These are both deductions, but both involve different things, so I wanted to talk about them. We're gonna work backwards to get an idea of how these deductions were made.
With the first one, rogha had a baseline of his friend and his friend broke the baseline - that is the main way that rogha was able to deduce that his friend's baby had been born.
To make that deduction, the baseline must have been that rogha's friend is an active Twitter user - it is uncommon for him to not be active on Twitter for a day. So, when he was, rogha could have asked: what could cause him to be occupied enough to not be on Twitter? If he is normally active on Twitter daily, then normal, daily activities like work/school/etc. would not be the cause. So *something* not typical happened to rogha's friend that day - something that took up a significant portion (if not all) of the day.
This is where knowledge about his friend helps - his friend has a heavily pregnant wife, who could go into labor at any time, so his wife going into labor is a likely reason for his friend to not be on Twitter.
Of course, a different *something* could have come up, so there was some risk involved in this deduction, but rogha turned out to be right.
With the second one, baselines are not really involved. Instead, rogha is relying on an understanding of the culture & typical behavior of where he and his friend live. In their environment/culture, "pumpkin" is a very popular Halloween costume for infants (and toddlers) - so it is easy and readily available to buy (Note: I know from my experience how popular a costume "pumpkin" is and has been for several decades in the U.S., so for the purposes of this post I'm assuming this is the United States).
Of course, just because the costume is popular does not mean every couple with a baby dresses their baby up as a pumpkin - there are outliers. But is rogha's friend likely to be an outlier? This requires rogha to have knowledge of his friend and what other sorts of costumes are possible. One possible costume route is what I'll call the nerd route - while pumpkin is a popular costume in general, for nerdier people, dressing up as a character from a piece of media for Halloween may be the preferred choice over a more "popular" or "standard" Halloween costume (think pumpkins, cats, witches).
For a young couple and baby, a nerd costume example would be doing Han Solo, Leia, and Baby Yoda. Rogha must know enough about his friend to know that his friend is not a "nerdy Halloween" type of guy (or his wife wouldn't be interested in that at least), so he is more likely to go the standard, popular route - thus, fat baby pumpkin is a likely choice. Again, there's a risk in this deduction, but rogha came to the "most likely" conclusion both times and ended up being spot on.