Being pedantic, the modern engines that you're talking about don't run at 20,000 RPM either, they are limited to 15,000 RPM and I believe they basically never actually reach that limit, usually topping out at 12,000-13,000 instead.

When the engines did run closer to 20,000 they were indeed rebuilt much more often. I am not well versed in F1 regulation history but Wikipedia claims that before 2005 engines were not required to last for two race weekends[1], meaning you could rebuild the engine between each race weekend if you wanted to. At that time there was no RPM limit[2] for the engines and the iconic Ferrari F2004 supposedly maxed out at 19,000 RPM[3].

So maybe the comment you are replying to is referring to pre-2005 F1 engines :) I have no idea myself if a modern F1 engine could run at 20,000 and still be as durable as the current engines or if running at such high RPM inherently means bad durability.

1. https://en.wikipedia.org/wiki/2005_Formula_One_World_Champio...

2. https://en.wikipedia.org/wiki/Formula_One_engines#Engine_spe...

3. https://www.f1technical.net/f1db/cars/873/ferrari-f2004