But, retail licenses can be moved from machine to machine legally. I presume this was a single machine license on the original machine and it's now a single machine license on the one it was transplanted in to.
I still have yet to understand how Microsoft is handling licensing these days, but you're far from the first person I've heard report that they did a "drive transplant," with everything including the OS on it, and it firing up as legally licensed on the machine into which it was transplanted.
This is precisely why I have said, and will continue saying, that beyond not doing what I know to be illegal, it's up to Microsoft whether something such as this is OK or not. When I have a circumstance such as yours, where I presume all of the original conditions for the retail license are still being met (meaning it's running on one, and only one machine, if it was a single machine license) and everything comes up A-OK by all official Microsoft checking mechanisms, my due diligence is done. It is not my job to decide whether or not a license is valid and legitimate beyond my doing my due diligence, and that's within reason, to ensure I'm not violating the original license agreement. Very often, and this is one of those times, full information is just not necessarily available about "the origin story" of a given Windows 10 instance/license. I have never felt obligated to "toss it away" if I do not know every scrap of information about it if MS seems to be perfectly happy to let it run.
It's interesting that the machine I'm typing from says it has a retail license, which I find very difficult to understand how that would have come about. This is a cast-off from one of my clients that had bad case damage (it's a laptop) that I decided to "revive from near death" just for amusement's sake. I maintained this machine for that client from the time it was new, and it came with Windows 8 Home, which I, once it was clear Windows 10 was stable, upgraded to Windows 10. It's still showing Home, but Retail channel. I've never had a pre-built machine (this is an HP Envy 15 series) show as having a Retail license "from the factory" other than this one.