"Let" not working.

Yes, the KA developers for the editor replace "let" and "const" with var (handling errors by hand), and eval the modified code. They also do other "post-processing" to your code such as changing the way "new" works.

Its because theyre using an outdated library that hasn't been supported since 2018 :( I've put in a feature request for KA to move to the new version of that library, but I don't know if they actually will or not

I think this is the post that Dino is referring to:

https://support.khanacademy.org/hc/en-us/community/posts/201252450-Update-javascript-let-and-const-not-working