Since valves (depending on design) are happy at microwave frequencies, speed is not a problem.
Valves can happily be used as 'switches' which is what is needed for such a design however the 'need' to do compression in such a manner was probably not necessary at the time.
Pulse width modulated compression has been around a long time but more prevalent in the 'silicon' era. Doing it with a handful of Cmos chips would probably be relatively simple, maybe more simple than the PYE modules.
The main advantage is 'linearity' in that the switches are either on or off but there are 'side effects' such as switching noise that need to be addressed. Thus the 'audible' effect is totally different to other means. It is a matter of exchanging one set of problems for another.