since this topic is still being discussed and it admittedly isn’t entirely to be blamed on reading comprehension, I’ll try to explain why megumi’s domain doesn’t count as an “open domain”.
so both megumi and kusakabe refer to the former’s domain as an incomplete, not open domain. this is important, and no, it’s not because they have a “conservative” mindset that makes them disregard the idea that megumi is approaching domains differently, as some people claimed.
there is a slight translation or rather semantic issue in the english text. when they cast their DE, sorcerers use barriers (結界, kekkai) to cut of the space that they want to expand their inner domain into from the actual surrounding space. so these barriers aren’t just boundaries but rather territories/zones, where sorcerers can operate individual rules including a DE typical sure-hit effect. however, there is another term in the manga, which is 外殻 (gaikaku) which can be translated as a domain’s “shell”, that is the actual boundary of a domain. so when a sorcerer expands their domain, they close of this shell.
now, what makes an open domain impressive is that it doesn’t need a closed shell. in fact the shell doesn’t exist, which is compared to painting on air without a canvas (basically an impossible feat). this DE still has a sure-hit effect though without the disadvantages of a closed domain (for example despite being a master of barrier techniques, tengen couldn’t manipulate and dismantle kenjaku’s domain simply bc it lacks this boundary/shell).
as I said before, megumi’s domain is an incomplete domain specifically because he struggles with barrier techniques (which in its core heavily relies on visualization). this makes it hard for him to cast his domain by himself, because he can’t set up and close the shell of his domain properly. which is why he relies on existing barriers and closed off spaces instead, like the gymnasium, where he can cast his domain into. but this results in an incomplete domain which lacks a sure-hit effect and results in a boost of 120% for his technique instead. it’s still an incredible move, but it isn’t a full fledged domain (which open barrier domains are!).
so basically, learning to omit the shell of the domain requires the understanding and mastering of a regular DE first. just bc someone can build something that looks like a plane, doesn’t mean that it actually functions and flies like one.
now, as I said before, 80% of a sorcerer’s skill relies on their innate talent and intelligence, which megumi has plenty of. combined with the fact that sukuna has used malevolent shrine multiple times in his body, it is very likely that he got the necessary understanding after shinjuku showdown.
(as to why gege gave him such a move to begin with: he said that it’s because he gave yuji the Black Flash before, which made him want to give Megumi something too. but a full domain would have probably been an overkill so early into the story, that’s why he got an incomplete domain instead).
