Much (most?) of spam filtering is done using DKIM/SPF/DMARC/ARC, domain reputation, and IP reputation, none of which require access to the contents.
I'm not saying that you should go and make all your email subjects "Free herbal viagra" or whatever, but modern spam filtering algorithms revolve somewhat less around those kinds of trigger phrases than they used to.
Also, spam is by definition sent in bulk, so as long as the emails are hitting either some accounts on other email providers or else are hitting some honeypots on ProtonMail then you still get the benefit of being able to have the sender blacklisted based on the contents.
This is for external mail only though. Presumably they don't have a bad problem with internal users sending mail to other internal users. I'm guessing any that did would get found quickly via reporting or heuristics.
Also, protonmail can see the contents of the email "in-flight", so they could filter spam at that time.
I've only used protonmail briefly and didn't notice any spam filtering features.