As titled. Why do people say pushups are useless for muscle growth if you can do them in high rep?
Why not just slow down the tempo and making it hard enough that you can do only 10 reps? Why is this not an effective (and safer) strategy for muscle growth?
There is a meta-analysis by Schoenfeld that discusses the effect on hypertrophy when alterating repetition duration. They found that hypertrophy is similar when training with repetition durations ranging from 0.5 to 8 seconds, as long as sets were performed close to failure. They also found that training with a repetition duration greater than 10 seconds is likely inferior for hypertrophy.
Thus, slowing down your repetitions or even adding pauses can be used to progressive overload your bodyweight exercises.
On the other hand I need to adress that in all of the studies discussed by the Shoenfeld meta-analysis both the slow rep and the fast rep groups never worked outside what most people would consider the 'hypertrophy range'. The fast groups generally never went over 30 reps in a set.
Therefore, to directly adress your question: It is certainly questionable whether slowing down your rep tempo for an exercise you can do 50 repetitions of is an effective way of progressive overloading. If you would do the repetitions so slow that you can only do 10 reps as you write in your question, then the repetition duration would certainly be beyond 10 seconds, which is likely inferior as pointed out.
You need to progressively overload a muscle for stimulate growth as your body adapts to the movement. Once you slow down enough, you'll be performing (essentially) a static hold. How do you progress from there? Longer times will increase your endurance but not so much muscle growth.
Slowing down works for a while but at some point you need to increase the resistance with more weight. This answer has extra information about stimulating hypertrophy on a muscle.
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.
