I'd be very curious how a WP plugin is managing this.
Third party hosting building in caching, normal in-line caches (like Varnish), nginx/apache caching, sure - I get those, and they behave how you say. But a WP plugin? I'm curious how that would bypass PHP & WP entirely.
On the other hand, this trick was mostly useful years ago when shared hosting plans were really underpowered.
As of today, with improvements of both PHP 7.x and general server performance, even a cache system that uses PHP very early (before any DB connection) to load a pre-rendered html cache file goes a long way in terms of number of pages being served.
Actually, if this become your bottleneck, the .htaccess trick won't help a lot, and you would still look in tools like memcached, redis or reverse proxies to improve things.
I think a lot of third-party hosts provide this out of the box, maybe even using their own CDN. WP is so popular that it's be difficult not to find a service that handled it all for you so long as you have room in the budget.
If you don't know how the cache plugins do this, it's pretty obvious you are not very knowledgable about WordPress site operations. Please stop spouting falsehoods here.