Mastodon and Lemmy are both built on top of the same protocol: ActivityPub. That said, they provide a substantially different user interface. Mastodon is focused on microblogging (basically, a Twitter clone), where you follow individual users and boost (retweet) or like posts. Lemmy is focused on communities (basically, a Reddit clone), where instead of subscribing to users, you subscribe to communities. There is some interoperability, but it kind of like a Spanish person trying to speak to a Portuguese person at this point.

Long story short, even though the goal is for all content to be accessible regardless of which platform you are using, there is a lot of content that can only be understood if you are using the correct one.

It is definitely inducing cognitive dissonance. The developers are simply are communists trying to collectivize social media, and that will live in their heads rent free as long as they stick around.