A 90 year old lady with mild htn but prior eschemic strokewoke abruptly, but late. She was confused and agitated for20 minutes. Was digital home BP of 189 real?

Quite possible. Although it could've been as a response, and not the cause. It's the chicken vs egg syndrome...
Probably. The BP of 189 systolic in an agitated 90 year old with hypertension is probably real, but it may be a symptom of the agitation and not a cause of the confusion.
Probably. The problem with digital BP measurements are reproducibility. To confirm one can measure 3 measurements 1 minute apart to confirm precisions. Then once bring and corroborate with a manual cuff at the doctors office.
Get evaluation ASAP . No point in guessing...Head to er as it could be minor or major.