It rang a bit hollow that hades could just reactivate robots from hundreds of years earlier which had been sent 'kill' codes and had been buried for so long.
I realise it's probably for story progression reasons but it just felt a bit false
The idea I believe, is that given there were hundreds of thousands of the robots globally, some were still viable.
This isn't stated explicitly but implicitly.
Note:
We see broken robots beyond repair thus we know many really are out of commission
We know they were just "shut down" thus it seems fair any still viable could be deactivated with another signal
HADES uses same transmitters that shut them down thus would definitely have access to right codes/frequencies
The robots can self repair and replicate: thus even if only a few remained viable that would be enough for a start
That said you're right there is also some classic character relevance and narrative progression at play too (and maybe a few consistency issues):
The robots we see awakening look identical to those we see Eclipse gain access to from the Horus and other sites: can't they self replicate/repair? Why not just use those since HADES already gains control of them? (Could be argued gaining access to more quickly via transmitter would speed things up for HADES)
Everything happens convieniently close to GAIA prime and Aloy: we only see robots awaken there and threaten that region (to be fair this is classic relevance but clearly if only a few deactivated robots were viable they could have been outside known territory)
I guess overall it makes sense as a plan in context and of course it has to all take place in the one (large) location the game is set in. Making it turn out core bunkers are there does go s long way up making it credible from a relevance point of view.
I'd have liked the awakening robots to be clearly new model and it noted the deathbringers and other robots already secured by Eclipse lack the crucial bio harvesting and replication capability to really make it obvious why HADES had to go for the transmitter.
A little foreshadowing some of the bio-harvesting robots could remain cable would have been nice too.
That said the fact we see viable robots bring secured from the Horus site dies in effect demonstrate this it just doesn't clarify if they an bio-harvest or not.