This is exactly what I've been wondering... The developer page doesn't really provide much insight into things either. Other than their development environment is based on React Native...

From last year announcement, this is custom linux.

After skimming TFA, I came here wondering the same thing... what is it and how is it different than Android, Linux or other alternatives. I realize marketing writers think they must always lead with the usual vaguely positive puffery but it has no concrete meaning and contains no actionable information.

And if it runs APKs from the Amazon Appstore on non-Android Linux, what tech are they using to do it?

I don't think it runs APKs. They'd probably market that feature more prominently if it were the case. They have their own format called VPKG.

https://developer.amazon.com/docs/vega/0.21/app-submission.h... "You must have a VPKG file available as an app binary."

This docs for FireOS, don't confuse with new Vega OS