It is, but note the definition of existence in the specs: Existence
In particular:
The default existence constraint, if none is shown, is {1…1}.
This to me means that if you explicitly state protocol in the adl but do not state anything about its existence, it becomes mandatory, even though per se it is not.
If I remember correctly, we considered changing this or publishing an erratum for this a very long time ago, but then did not.