His point being is that Morrowind had tons of Daedra without the oblivion gates, so the whole "there's not a lot of daedra because of the closing of the oblivion gates" excuse doesn't really fly. I think that's what he meant anyway.
The oblivion gates aren't the only way Deadra arrive on Mundus. The oblivion gates just made it considerably easier.
Originally, keeping the Dragonfires lit kept the number of Deadra slipping into the world at a manageable number, the end of the Septim line proper & extinguishing of the Dragonfires was what allowed the hordes of Oblivion to arrive on Nirn in mass, through oblivion gates & other means.
Spoiler But when Martin died, smashed the Amulet of Kings & become Akatosh's avatar he sealed the barrier between Mundus & Oblivion permanantly. So it's actually harder for Daedra to be summoned, or appear of their own volition than it ever was.
Mark that for spoilers too, just in case.