That's the theory, but it's not like they use strict markdown. Markdown in the first place is a very simple and limited format, so everyone has their own syntax-extensions and tweaks, and obsidian is no exception. So even if you lose the data themselves, you could lose a significant amount of ability to work with them, if somehow obsidian becomes unusable.

And if you build on plugins, this sometimes happens even now here and there. Plugins becoming unusable because of an update is still not uncommon. Their developing stopping for whatever reason is also a bit of a problem.