With 'normal' DNS, UDP with the default and TCP is used if the packet size becomes too large. There are other TCP-only variants such as DoT (DNS over TLS) and DoH (DNS over HTTPS).
I don't think the performance would matter much with some basic caching (or even just OS-level caching), but there is limited memory in an ESP so maybe that is it. I have never noticed issues with DoT and DoH which are theoretically much heavier protocols.
That’s odd because DNS is the quintessential UDP-based protocol. “From the time of its origin in 1983 the DNS has used the User Datagram Protocol (UDP) for transport over IP.”. DNS over TCP was only introduced as a later addition (admittedly, in 1989).
The author cites it as performance reasons, but at this scale, even the uplink to cloudflare, would be negligible, no?