The resistor network output voltages do not match the optimal modulator input voltages, and they do not have to match, as long as the modulator input voltages are within the range.
The antenna RF inputs of the TVs work under various signal strength conditions anyway, like due to range to various transmitting TV stations or good/bad weather conditions.
So there is an automatic gain control (AGC) block in the TV signal path that will amplify the signal by the necessary amount to get the demodulated composite video amplitude to the expected level.
For PAL signals, the expected composite signal amplitude is such that there is 300mV between sync tip level and blank/black level, and thus the white level is 700mV above black level.
As the RF signal uses negative amplitude modulation, the sync tip level voltage sets the maximum RF carrier amplitude level.
So everything in the receiver is relative to the maximum received RF carrier amplitude.
Even if the modulator input at sync tip level does not go to the rated minimum voltage of 2.6V but stays above it, it is still the strongest level of RF carrier it can produce at sync tip, so it simply looks like it was just sent with weaker RF power than it would be if it uses nominal 2.6V level.
All that matters is that the signal is DC biased with high enough DC voltage level to operate at the linear range of the modulator, between 2.6V and 3.4V.
And what the adjustable resistor does, is to allow setting the DC bias point, and the AC signal amplitude at the same time, simply to a level that works good enough to see a picture on TV. The lower the trimmer resistance, the lower the DC bias, but lower video signal amplitude too. Also the 24S10 has TTL output stage so it cannot push out much current, but it can pull more current, so I suspect that the trimmer resistor is used to actually set the voltage level of near white level, from where the 24S10 outputs can only pull the voltage down to black and sync levels.
So that is why the actual AC voltage amplitude at the RF modulator input does not matter much. It just has to have the correct relative amplitude levels, 30% for sync pulse amplitude and 70% for video to have a 100% composite video level.