the thickness and size of the steel plate and the fact that it
was screwed to the stud with big honking screws gave me no
concerns about the support of the fixture and electrical box.

Sorry, I guess I misunderstood your initial post:

"When I removed the plates I found that the electrical boxes were not
attached to the studs"

I got the impression the box was just "floating" somehow other than the
front strap for the fixture mounting. I couldn't see a mounting bracket in
the photo, but maybe it's there in the bottom of the hole?

Either way, as long as the box is secured somehow, you should be fine.

Anthony Watson
www.mountainsoftware.com
www.watsondiy.com