Yeah, that’s true. It’s on purpose:

Many services block addresses from anonymous email services, so we've made the conscious decision to not provide anonymous addresses on the port87.com domain. We do understand the value of maintaining your privacy through anonymous addresses, though, so we do not block anonymizing services, and we encourage you to use them with Port87 to enhance your online privacy.

- https://port87.help/#support-for-anonymized-email-services

If you have your own domain, Port87 supports that though. So you can use that without a shared username between your addresses. Basically the same concept as a catch-all through the other services you mentioned.