Pretty much, yes. Other providers are small enough (except for maybe Microsoft for business) that it's generally their problem if they accept less than Gmail.

If any of them hold the mailbox of someone that you or one of your users needs to reach, it quickly becomes your problem.

Now suppose you contact that server and complain about being rejected.

Wouldn't it be ironic if they respond like this: "We receive e-mails from gmail just fine; fix the problem yourself, or use gmail".

This is how self-hosted e-mail people throw each other under a bus and let gmail win, while pretending to hold self-hosting as a cherished value.

(They would most likely be right about having to fix the problem yourself, unless they imposed some locally authored and highly unreasonable/dichkeadish filtering rule. The superfluous rhetoric about gmail would be almost as obnoxious as their rule, though.)