if it's not the real Ilya in the shower, I don't see the point of Cliff's line "You get any sleep last night? You're a fucking beautician" the next morning, insinuating that Ilya got laid, like that seems like an intentional ironic nod to the fact that Ilya didn't get laid like Cliff is assuming but jerked off alone, especially because Ilya gives a very awkward nod/laugh in response. Idk I am also partial to Ilya actually pathetically jerking off alone lol, though the discrepancy with the showers would definitely be a weird continuity error if so.
So, I would say it's still possible that Ilya went home alone and jerked off. Like, I personally still very much think that is what happened. That's what happens in the book, something we know Tierney follows when he can, and I think in all likelihood Tierney did include cliff saying this and Ilya responding this way to suggest Ilya didn't go home with anybody in the show either.
But, assuming he did, he went back to his hotel room and jerked off. The shower we see Ilya jerking off in is Shane's. We are given the exact scenes necessary to recognize that shower as Shane's and not Ilya's hotel shower. I think it's extremely unlikely that's a continuity error, because Tierney would have basically had to go out of his way to make that error. There was no reason there had to be a shower sex scene specifically in the opening montage, giving us a look at Shane's shower. There's no real reason Ilya has to look at himself in the mirror in the hotel, giving us a look at his hotel shower. Tierney could have included a different sex scene in the montage, and we could have seen Ilya brooding elsewhere, or at least not with such a perfect view of the shower rod behind him. I think it's obvious both of these scenes were very intentionally included.
And based on how the club scene is directed, and how Shane's sex scene with Rose is directed, it is also clear that 1) what we are seeing is not a literal depiction of events, 2) this is Shane's moment, and the story is currently focused on Shane, and Shane's arc.
The other thing is like, let's consider why a scene of Ilya jerking off would be included. Not a fantasy Ilya that Shane is imagining - the real Ilya jerking off. What's to gain from including that?
We already know that Ilya is pining for Shane. We already know that Ilya only wants Shane. Hell, in the montage of the same episode, we even already know that Ilya has lost interest in fucking other people.
Unlike Shane, Ilya doesn't have any major revelation to be making about his sexuality in that scene. He doesn't have any other major revelations to be making either.
There is nothing for the narrative to say with Ilya jerking off, except quite literally that he did not go home with somebody - which again, we already know Ilya isn't actually interested in going home with anybody. We already know he only grabbed that girl to dance with specifically to upset Shane.
We'd get absolutely nothing out of it - out of a moment that is more evidently than most trying to make a clear point. If that's the real Ilya jerking off, what would be the point?
A fantasy Ilya serves a purpose. It shows us the exact revelation Shane is having, which is that even with a girl he really likes, a girl unanimously considered attractive by his fellow hockey players, the "right" girl - he can't enjoy fucking her. He has to imagine Ilya jerking off to get off himself.
And again, look how differently the two scenes are directed. People act like those scenes are parallels - Shane thinking about Ilya, Ilya thinking about Shane - but they're not. They're not filmed the same way at all, they don't mirror each other at all. The way the camera slowly steps up to Ilya like a person, from the other room until it's right up in his face - why? That is not an arbitrary choice. We don't see anything like that anywhere else in the show. Why is it here? If that's not being used to reveal something to us about Shane - a creeping realization that becomes unignorable about his own sexuality - what is it being used to reveal about Ilya? That he's jerking off? That he's thinking of Shane? We had no reason to guess otherwise...
I think Tierney wanted to convey to the audience Ilya went home alone, but he couldn't have done that without changing the club and Rose sex scene, and without taking the attention away from Shane during a pivotal moment of his character arc, and so he decided the fantasy Shane would have would also allude to what Ilya went home and did, and this created a bunch of confusion.
But I stand by that we are not seeing the real Ilya jerking off. We are seeing Shane's fantasy. I think it's definitely implied Ilya went home alone and did the same thing - but we don't see it.
If I'm wrong, that scene is not nearly as impressive as I thought.