Proxmox is not a hypervisor. It is a Linux distribution. As such it has a web server, kvm, zfs, and many other pieces. Maybe the acme client is built in to the web server. Maybe the acme client is built into their custom management software. Maybe they're just scripting around certbot.

I do tend to find that I need multiple services with tls on the same machine, such as a web server and RabbitMQ, or postfix and dovecot. I don't know how having every program have its own acme client would end up working out. That seems like it could be a mess. On the other hand, I have been having trouble getting them all to take updated certificates correctly without me manually restarting services after cert bots cron job does an update.