It looks like they are making their own cloud, with such a little team?

Also we should be using Matrix but it doesn't have half of the features (no channels, no mini apps etc).

Also I wonder whether compiling JS to native code is worth the hassle or not. In browser, it would slow down page load, but here you need to compile only on deploy.

This is barely a cloud, since it only serves one particular purpose and everything runs in V8 isolates. Although they would need to ensure they're appropriately capping each tenant's compute and bandwidth; the article mostly doesn't get into that.

Fully AOT compilers for JavaScript are basically research projects rather than production-ready platforms, because the extensive dynamicity of JavaScript's semantics makes this a hard problem. (The most mature one I'm aware of is https://porffor.dev.) But even if that weren't the case, the thing Telegram is doing requires V8's low-overhead tenant isolation, so they're bound to V8's architectural choices. V8 does have APIs for code caching and startup snapshots; Telegram could be generating those at deploy time.