Does F6X Solar charges by light while shutdown?

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I have the Garmin Fenix 6 Pro solar, not the X, and wanted to throw my hat into the ring. 

I let my watch run until it died, or at least would not turn on fully. It would just give you the Garmin logo, then say low battery and then shut off all in 5 seconds. I left the watch alone for a whole day after that, checking occasionally to see if it would start up and it didn’t, just trying to drain a every ounce of battery life out of it. The next day I then placed the watch on a bright window sill for a day (about 3 hours of solid sunlight, maybe more with spotty sunlight) and it didn’t turn on. I left it for a second day in the window, the watch got another 4 hours of good sunlight and at the end of that day I tried the watch and it turned on fully. I didn’t note a battery percentage, my widget just said “low battery” but it was enough to play around for a few minutes before I put it back on the charger to charge it fully. 

TLDR: Yes the Garmin Fenix Pro Solar charges from sunlight when it’s dead. 

FWIW, almost all kinds of rechargeable batteries will regain some charge from a state of being completely dead, if you leave it powered off for a while. It's the same with cell phones. And it's why if you have a dead car battery, you can sometimes just let it sit for 30 minutes and it'll get regain enough juice to start your engine. But it wasn't the sun that did it, in any of those cases.

I'm not saying the Fenix Solar can't recharge while powered off, I'm just saying that your experiment isn't very scientific, and the conclusion you drew from it was still just a complete guess. If it recharged, say, 50% of the battery, then I'd say maybe you're on to something. But to only gain a tiny amount over an extended period of time, it could've done that on its own without a solar panel.