You are right that joint intent documentation is to be completed at the time of application. However, you're mixing Reg Z (TRID's definition of application being 6 items) and Reg B's (a request for credit . . .). Joint Intent is not triggered by a "completed application" but rather by the (initial) application - the original request for credit.
The loan officer should have documented the applicants intent to apply jointly when they initially started the mortgage application - probably 2/28th or sooner. You don't have to have a "joint intent notice" to comply with ยง1002.7(d). The loan officer can ask about their intentions and simply document it. Maybe they've done that and are using the form sent on 3/1 to firm up what they documented.