He played 2 games in the regular season. He was great in the first game and so-so in the second one. He has then had 2 decent to bad playoff games and 1 pretty good playoff game. The hype was out of control and that's all some people are calling out (other than the one guy I agree is trippin).
To answer your question, the kinds of guys that end up becoming Ovechkin-level talents, i.e. all-time great players, generally do look phenomenal from the get go. It's hard to make a list of guys that did it in the playoffs because generally playoff teams aren't adding rookies to their team.
However, I'll give it a shot.
Makar was phenomenal in his first playoffs as one example. McAvoy had 3 assists in 6 games and was very good in his first playoffs. Guentzel, though a few years older, had 21 pts in 25 games in his first playoffs. Quinn Hughes in his D+2 had 16 pts in 17 games in the playoffs and was similarly fantastic. Nathan MacKinnon in his D+1 (same as Demidov) had 10 pts in 7 playoff games, showing the game-breaking potential it took a few more years for him to fully realize. Karlsson had 6 points in his first 6 playoff games, albeit after a long and productive season. Hertl 5 pts in his first 7 playoff games.
Obviously these aren't directly comparable because a lot of these guys had more of a full season to get up to speed, but the idea that young players can't excel in the POs isn't true, especially when you're comparing said player to people like Ovechkin, Kucherov, Malkin et cetera.
I only included active players too, the list is way longer if you look at all players. The point is that true all-time greats usually do look like all-time greats from the very start, even if they're starting mostly in the playoffs. I'm not even saying that Demidov can't be that, but it's totally fair to call attention to the fact that he's not doing anything close to what an Ovechkin-level talent would be doing. That doesn't mean he can't become Malkin 2.0, I'm sure he will be pretty close to that. But it's totally fair to call out.