Any exercise is better than no exercise. That said, for cardio exercise there are nuances that can affect you. HIIT is quick and stressing, and moderate cardio is slow and time consuming but gentle. Are the results the same for both?
High intensity interval training HIIT requires you to go all out in your cardio workout, but for extremely short periods of time, such as one minute. You then rest for a couple of minutes and repeat. A couple of cycles of this, and three times a week, and you’re done. You may not even do cardio for an hour per week, cumulatively. Maybe you do this for only 15 minutes a week.
Moderate cardio is when you go for a walk or ride your bike at a tolerable pace that allows you to talk. It is not painful or stressful but gentle, 30 minutes a day, 5 days a week. Weekly this takes 150 minutes or 2.5 hrs.
This NYT article quotes a Guelph university study that concludes that for lowering blood pressure, moderate cardio 5 days a week is more effective. That said the study was very small (23 men) and for a relatively short period of time (6 weeks), and involved obese men.
I’ll search for more articles and evidence.
HIIT and Hypertension : Improvement of cardiovascular functions
This study shows that HIIT with long intervals (4 x 4 minutes at 90% FCMAX) significantly reduces blood pressure at systolic (12 mmHg) and diastolic (8 mmHg) compared to MICT (4.5 / 3.5 mmHg) . And the level of reduction is similar to some drugs prescribed for hypertension. In addition, HIIT allows a better improvement of VO2MAX. Finally, only HIIT has improved endothelial function that allows better vasodilatation of the arteries. It is probably these improvements (and many others) that can reduce blood pressure.