I've come across at least two recent threads where people bought devices based on this statement, and assumed the factory image could be ULed directly through the stock webUI.
If you're changing the F-S frontend, perhaps add a search towards https://git.openwrt.org/openwrt/openwrt/ where the search word is the name of the device selected in F-S ?
We're kind of getting side tracked, the main "issue" was the "You normally do this via the web interface of the original firmware” writing at F-S.
Anyway, a while back I was considering creating a script that would extract and announce new devices getting supported, I had a look at the merged PRs, and noticed most of them used a template.
If you'd only pick up the model (cut out everything left of and including) the underscore and add "add support" to the search string (which is the title of the template), you might have better luck.
It won't work for older hw though, the template wasn't used back then.
Simply removing the vendor_ from the URLs you posted improves things considerably.