I have not tested this or done something similar. However, I concur with Mark's 2nd comment that it will be a "multi-record" operation, but for a slightly different reason.
The "starting in November" release-note really does not directly answer your question. It seems to imply, however, that an automations with an action step that modifies more than one record will be processed like a "multi-record" operation. Possibly, the internal implementation is similar to doing a grid-edit on multiple records and saving them as a batch (not one by one). If so, then the internal implementation for your follow-up email-notification would be the same as if you had done a batch grid-edit or a csv-import, i.e., a multi-record email.
As a precaution, you might use the same email template for both single-record and multi-record scenarios, if you can craft the text of the email appropriately to fit both scenarios.
