Yeah, this doesn't get brought up often enough. Honestly, while I don't necessarily disagree with it, I think a lot of the "social media is evil because it was engineered by psychologists to be addictive!" narrative is complete cope.
Cope, specifically, because we don't want to accept the fact that this kind of stuff is addictive on its own, that we are our own worst enemy; bot armies, evil corpos, and engagement algorithms don't help but they're not required. (That is, between your two theories, I think both contribute but it's more so the latter.)
I'm a pretty easily distracted person. I don't use social media at all. Yet I've been "addicted" on some level to this site, to news sites, to browsing Wikipedia, to traditional forums. So have plenty of others.
People don't want to face the fact that humans just really like having a giant source of stuff to entertain ourselves with, and are easily drawn into online arguments. Getting rid of the corporations and such would probably make a better Internet, sure, but it's not going to cure everyone's addiction.
Facebook pitched itself to China on the grounds that it will help China to survey and coerce toward social harmony better.
Facebook targetted teens who posted self deprecating comments for beauty and weight loss products ads.
It was no random that facebook became a hotbed for conspiracy theories, misinformation and lies. That part is result of their own conscious choices. They made it that way, because they wanted it to be that way.
Facebook moderation favored pro-genocide side of the politics in maynmar. Its own decisions made it instrumental to the events. The issues were solvable and visible, Facebook was the primary gatekeeper there due to contracts it had, but Facebook refused to do anything even after it was crear what went on.
This is exhibiting a lot of software guy detached from reality. The reality is the teen girls kill themselves for being "too fat" because bullying engagement makes money for big commercial social media sites. Somebody else on this thread claimed to be somewhere addicted to Mastodon sites.
In principle they're comparable. In the real world, no high school kids are going to harm themselves over an argument about Mint versus Ubuntu on a 100,000 subscriber ATmosphere site. It's like comparing the Dominican cigar shop selling hand rolled cigars with Philip Morris. Unedifying.