I don't know about the not public betas, but as soon as there was a public Windows 10 TP and you could swap IE's engine to Edge I'm pretty sure they had "Edge"

I don't know about the not public betas, but as soon as there was a public Windows 10 TP and you could swap IE's engine to Edge I'm pretty sure they had "Edge" at the end of the user-agent, but they had the rest of the Chrome UA so most of the websites out there considered it as being Chrome. I may also be completely wrong :)