That sounds like I am totally wrong about the rate limits then. Which does make sense, because my interpretation just seemed flawed.
So, if I may just clarify, with another contrived example (Sorry!)
Lets say I've decided that Route53 is charging too much at 50c/domain, and I'm going to give away DNS hosting for nothing, but make it up in volume. To do this, I'm going to need a bunch of nameservers, and a bunch of machines.
Because I like scalability, I deploy one new machine per day, with the FQDN of {{random uuid}}.superawesomedns.com, but because I can't write APIs, every host needs to expose a https service to the end user.
I use the standard letsencrypt client, and http auth, to get a unique key and cert per host. That then has a cronjob that renews the cert whenever required.
Because I'm not hitting the 20 hosts per week, that should be fine? Or, am I going to hit another rate limit somewhere.
Re-reading your original post, in the other thread
That's obviously where my disconnect happened.