this is only true for social media that promotes particular content, which means it is not true for signal, telegram, whatsapp, wechat and many others that just share messages as the users intended.
case in point, in this wikipedia article wechat is clearly categorized as social media, even though it's messaging component is no different than whatsapp, telegram or signal, which are "maybe" social media.
in summary, some people call them social media, and some don't.
Enough of these utilities engage in this attention-grabbing funnel tactic that the effects of their algorithms far exceed the boundaries of their own products, and reinforce each other as well.
the distinction is where the steering comes from. on facebook the steering is affected by the algorithm, which decided what messages you see, while on telegram it is not.
on facebook, twitter, youtube etc, i can be confronted with extreme content that i didn't ask for.
on telegram, whatsapp, wechat, etc, i only get content that i explicitly asked for.
we can argue how much surveillance is acceptable or necessary. but that is a different topic.
the purpose of moderation is to avoid discussions being sidetracked by extreme messages or bad faith replies or other noise, so we can focus on the topic at hand.
but whatever happens in these groups does not affect the discussion in the groups i am joining, so their moderation is quite irrelevant to me and provides me no benefit.
this is different on public forums like hackernews where i can access all content and expect to have a civil discussion everywhere. hence moderation here is necessary and welcome.
but if telegram were to come in and try to police speech in my private group then i'd be very upset and look for a different service with more privacy.
what you are effectively advocating for is that such private groups should not be allowed to exist because there is no way to control what is being shared in those groups.
yes, we want to stop these things, but blaming the tools for allowing them to happen is the wrong approach. either we allow private conversations or we don't. if we don't, then noone can have private conversations anywhere. that is not something you should want.