I think you nailed this one. Poser was my first thought, but a poser is someone who fakes an interest to fit in. Dilettante has that gatekeeping aspect to it, which seems important to OP's description.

I agree with this, but it lacks the ‘pretentious air’ about the individual. A charlatan or poser equally partially describes this. This actually seems to be a missing term.

Yesss yes yes yes I think you've nailed it thank you! I didn't know this word, I've read the definition and it suits quite well.

I second pretentious.

I think pretentious is the closest match but it's still lacking something else. Another commenter suggested two words instead of one, I guess that might be the way to go

OP's hypothetical person is explicitly assertive. Pretentious doesn't convey assertion. You can be pretentious internally and keep it to yourself.

Or a "fantasma/casper"

En Chile es "alumbra(d)o", que se da "brillo" o importancia.

I always called them "arrogant for no reason"

Was gonna say, I'm not throwing stones in this glass house.

I don't think a braggart is necessarily lacking the skills/knowledge they claim to have

I think the point you might be missing from the OP, is that the person in question doesn't actually possess this knowledge or skill, but instead of being humble and admitting the gaps in their knowledge, perhaps asking a question or two when they don't understand, they just plow ahead.

I know people like this. It's not malicious I don't think, maybe it's more of a defense mechanism? They don't want to look uninformed, so they try to fill the gaps in their knowledge with assumptions. Maybe they're right a decent amount of the time, or close enough to fool people who aren't knowledgeable on the subject. But people who are knowledgeable, will recognize what they're doing, and the lack of humility comes across as arrogance, even though that may not strictly be the case.

It's normal to know a lot of stuff and like a wide variety of things. But you need to be humble when you don't know something.

When I don't know something for sure, I like to say "I think it's this way, but I could be wrong." Or something like that. Or heck, if someone else has the knowledge and skills I don't, I'm right up there asking questions, I want to learn. Maybe I'll try to impress them with what little I do know on the subject by commenting or asking questions in a way that shows I understand what they're talking about, because we all want to have self worth. But faking it isn't cool, and can be harmful, or just annoying.

Did you maybe mean supercilious?

Edit: It seems like OP wants something like "supercilious incompetent"