What does modern mean in this context? I see this word being used a as a positive adjective a lot recently to describe rewrites etc but it is unclear to me what it actually implies
The term is overused, yes, but one way it can be understood in wasm's context is that there is now a generic platform/target and we have consensus on it. It's hyped, the inertia is there, so all of it is going to happen. And in this particular case it's a good thing. :) Probably soon we can stop rewriting everything in javascript.
By that I mainly meant that it runs on today's systems and maintained languages and toolchains today can target it. Sometimes in discussions about WASM, I see people bring up ancient bytecode formats in a way that makes them sound like usable alternatives.