Just writing a javascript runtime alone isn't just a "handful of engineers." WebGL stack? WebRTC? Layout engine/compositor? Notifications? You're kidding me, there is no small team in the world that could ship a browser stack end to end.
They've managed to actually get GMail to render - a not insignificant task: https://www.ekioh.com/devblog/full-google-mail-in-a-clean-ro...
The timeline on this page alone is in years: https://www.ekioh.com/company/#team
From what I understand, that team is relatively small in comparison, but actually does have this widely-ish deployed. So it theoretically could be done with less... but it's still insane to even consider. This isn't simple, and anyone who's trying to imply otherwise is wrong.
The other commentator you're responding with also never discloses they worked with Apple previously, while pretty much endlessly pumping up their work here. shrug