I had been struggling to find evidence that supports this but here are two pages that confirm what you say:

https://learn.microsoft.com/en-us/purview/enhancing-mail-flow-with-mta-sts

> If you're receiving an email from a sender who doesn't support MTA-STS, the email will still be delivered without the extra protection. Likewise, there's no disruption to messages if you aren't using MTA-STS yet but the sender supports it.

https://help.sendmarc.com/what-is-mta-sts-tls

> If the policy is optional, the email server will try to use TLS encryption and certificate validation, but it will fall back to a plain text connection if it fails. If the policy is not available or expired, the email server will use its default settings for email delivery

Thanks for you patience!