Not necessarily. Plex does not hold the private key for their intermediate certificate authority ('Plex Devices High Assurance CA2'). They use a DigiCert API to sign certificates under the 'plex.direct' domain.
There's no reason not to use LE. You need to make sure that your device is accessible under the given subdomain for a certain period, and it may not always be trivial. But there are ways to do that.
I tried to set up Let's Encrypt for this purpose for a personal project (to be given away as a gift), and concluded it was basically impossible as things are.
You don't want your users to have to figure out how to open their firewall enough for an http server to be publicly reachable just to use your device, especially when it has no other need for it. It would need to be reachable every 60-90 days to renew the certificates, which is frequently enough to mean "always".
The ACME protocol does have a DNS-based challenge mechanism, but it requires putting a token in a TXT record at an administrative sub-domain of your actual domain (e.g., _acme_challenge.example.com). As far as I can tell, none of the free DNS providers support or enable this (nor does certbot, without writing your own hooks). I don't want to eat a domain registration fee in perpetuity for a gift.
Sadly, there's no ACME challenge that can be satisfied solely by the actual thing you're trying to assert, which is that you control which IP address a given domain name points to.