It has to do with whether hanging out with others drains or fill you with energy, mentally.
I love hanging out with people, it fills me with joy, and I'm great at talking with people, I'm good at cracking jokes that causes a group to laugh, I can hold speeches just fine, or perform in front of a group. But once I'm done, I'm exhausted, it's like holding my breath, I need to come up for air and be alone, even if I enjoy being with other people.
These are hypotheses that I wouldn't rely on to give advice. It could discourage someone who is dissatisfied with themselves from trying to change. Who says it's impossible? Your personal experience, or observing people around you, or a study on 1000 people?
Humans are not meant to go through the world alone and modern life can be incredibly isolating. It takes effort to build social connection but it is well worth it.
Introvert does not mean asocial or antisocial. Social connections are a fundamental part of every human's mental health. Maybe some disorders might change this up, but being an introvert is not that.
I've been guilty of this in the past regarding sleep. "I don't need more than 6 hours of sleep per night" I kept telling myself and others, and I ran on that schedule for decades. When I started forcing myself to sleep more, I started to feel even better, and now I'm hovering around 8 hours per night and feel so much better. I didn't even realize I could feel better by doing something I didn't think was necessary, but it did improve me.
I've also done this related to relationships, where I found myself fine with being by myself for long stretches of time. I didn't really see any problems, and when people told me I have to see other people, I also used the "maybe I'm not "normal" and don't need it like others?" argument, which is what I told myself too. I didn't feel bad, but started seeking out more relationships anyways, and got so many benefits and became happier because of it, that I can't go back to being all alone again.
I'm not saying this applies to you, but maybe it applies to others who read. It's easy to get into the trap of lying to yourself (I'm guilty of it multiple times), with all kinds of reasons. If you can see past that, you can become happier, even if you're not miserable right now.
That's typical behavior you see on social media Twitter. People say something reasonable, but then someone replies "how wrong!! this does not apply in edge case XYZ!", yeah, obviously. Just ignore it if you believe it doesn't apply to you.
If you feel this way, please try to see it as "telling somebody with depression to pick the activity that seems less painful to do and go do it".
It is possible for that depressed person to not be able to do it yet. However, unlike your 'cheer up' example, this is actionable advise that is very likely to (slowly) help lead that person out of the depression if they manage to do it.
I'm an introvert and it works like that for me. I hardly ever feel like socializing. It drains me when I do it. Yet I still enjoy it when I'm doing it and it has a positive effect on my mental state (up to a point, of course).
The key is to find the right balance, and to be aware that there's some "training" to it: the less you do it the harder it gets. Don't give up though because then your mental health may suffer.
I am happy with a small handful of friends. If I'm going to add new ones, there has to be a significant overlap in interests that makes the company particularly enjoyable and non-draining. Probably a more interest-specific/targeted approach than Meetup (although maybe Meetup is capable of that, I am not familiar with it)?
Maybe I should try it out and see for myself, though! Could be one of those things that's pleasantly surprising.
[1] "In my case i signed up to Meetup and just attended random stuff until people stuck. This usually involves hiking, pubs and bars, restaurant nights out."