Optional memory safety is, when you can opt an entire project into a "strict" mode, and this becomes trivially verifiable by others. I imagine that's the goal here.
Optional security is a problem only when you need to remember a million different rules and gotchas, because you will inevitably miss a spot. But if it's a global toggle, it's pretty good. "Use '-fmemsafe' for C/C++" is as tractable as "don't use 'unsafe' in Rust".
Yeah, as you note, library compatibility is an issue. But it's an even bigger issue when bootstrapping a new, safe language: you gotta implement the libraries from scratch, and you never really get to full parity with C/C++. Getting it done for the top 10 most-used libraries would make a spectacular difference in itself.
I should note that I'm not a huge believer in "saving" C/C++ as the memory-safe language of the future - I think there are lingering cultural problems around the standards that we had no luck overcoming for decades - but I also don't think the duo is going away any time soon, so might as well expend some effort on making it a safer tool.