"professional behavior" would make sense if both sides are getting paid.
If only the user is getting paid, and he wants support or something else from the FOSS author, why should the FOSS author be held to this "professional behavior" standard?
It helps me to remember in a public forum, whether my point is acknowledged by the OP or not, is only a small fraction of the number of people who may read and accept it. If the OP unreasonably denies or refutes it, in fact they may be silently judged accordingly.
No need to make your good point a hostage in your own mind to whether the OP will publicly accept it.
https://www.kernel.org/doc/html/v4.17/process/kernel-enforce...
... but keep digging anyway.