I really don't know if they *test* the ECUs (as a specific service), they're just the only two vendors mentioned here who do *anything* to/with the ECUs.
(people have sent in ECUs to GDE, had them return "working", but never heard what (if anything) was wrong with the ECU when it arrived in GDE's hands).
I would suspect that GDE rewrites the ECU *first*, and then does whatever tests they do. That's simply the more efficient way to crank them through...
If the ECU tests as "bad" after-the-rewrite, then it becomes "diagnostic time".
Just give them a call (or email) (and please report back with results...)
good luck
--dick