Your mainstream user doesn’t care about the underlying technology behind a product. They care about good UX/UI, good content, who’s on it and a supply of dopamine to keep the “must check social media” cycle going. There’s a separate group of people who use social platforms to stay up to date with the news. Think Twitter/HN. It’s not so much about dopamine but necessity to stay up to date on things/discovery. There’s really no shortage of platforms out there. I don’t know if we want to bundle general chat applications like Discord/WhatsApp into this as they’re more closer to SMS than Instagram or TikTok. All the talk about “federated” or “decentralized” infrastructure matters little to people outside tech people and the tech enthusiast crowd. Programmers wanting widespread adoption on a platform should be focusing on the product itself and value to content creators versus the nuts and bolts around the product.
I think you might be conflating what are the most profitable social media business models with what people actually want. It's the difference between a lucrative something that addicts or traps, and something people genuinely enjoy or believe in.
I think there is a growing amount of people who want to get out of the social media Skinner box, and there are also growing amounts of people who believe in new and alternative technology like this.
Sure, these platforms might not appeal to someone who uses Facebook like you would a slot machine, but I know plenty of non-tech people who use things like Linux, Firefox, Android/LineageOS/GrapheneOS, etc for moral and ideologically appealing reasons.
The problem is that there is also a steady supply of people coming in who don't give much of a damn about any of this, and just want to do what their friend group does. They might get more aware later, but by then there's already a new generation out there making the same mistakes.
Unless we can push ethical (and FOSS) software over the point of critical mass, it's a never-ending losing battle. And even if we get there, the battle for daily attention is much fiercer than, say, Blender or Godot permanently winning mindshare among professionals. Trends are fickle and even just by not being the next best thing, we can lose the masses to the next best Skinner box.
I think you're misreading the comment. It's not saying it's becoming relevant because it's Foss, just that mastodon is more culturally relevant and is Foss.