Why would they be joking? There’s really not a big difference between how their mobile and desktop apps work and what’s possible in the web. It can fetch the keys from my computer or my phone just like their other apps work, and store the keys and whatnot encrypted in temporary local storage, just like on the phone. WebAssembly could allow them to share the code and retain similar performance.
I honestly don’t see an issue here. If they need help, I’d be happy to lend a hand.