As I understand things, this change would more formally open the door for single-transaction blocks. (Zero-transaction blocks being a thing at times in Bitcoin's history, as, of course, non-filled blocks.) Am I right about this OP_RETURN change insofar as there would be no coded limits on transaction size and it would only be limited by block size?
No, the transaction size standardness limit was not meant to be lifted with this pull request. While there is an idea to later lift also the number of OP_RETURN outputs per transaction, this would not affect the transaction size standardness rule.
What is this standard, where is it defined, and how is it enforced?
The AI associated with a google search on the topic quite clearly communicated (to me) that there are no such limits, but it well could be one of those 'ai hallucinations'.