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.
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"Even Microsoft was kinda masquerading early versions of Edge as Chrome"
I would assume that was more to keep the tech press from seeing it.
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 :)