The rule is that "using a resistance so light that you could do a very large number of reps is suboptimal for hypertrophy", not that "doing high numbers of reps is suboptimal for hypertrophy".
If you can do 100 pushups, slowing them down to the point where you can only do 10 doesn't change anything. It's still a resistance so light that you could do 100 (normal speed) pushups, so it's still suboptimal for hypertrophy.
Or, looking at it differently, you could say that a set that takes you more than, say, 2 minutes to fatigue is suboptimal for hypertrophy. Doing 100 normal speed pushups in 3 minutes is then no different from doing 10 tempo pushups where each rep takes 18 seconds, for a total set time of 3 minutes.
You need more resistance, and tempo work does not provide that.