tl;dr; Nope.

Assuming that HIIT workouts are 100% vigorous activity (unlikely), then a "few" instances would only add up to around 24 minutes of vigorous activity, which is far short of the minimum recommended 75 minutes of vigorous activity.

If you are short on time then performing HIIT for 15 minutes five days a week will get you much closer to the minimum requirements.

4-minute HIIT run (30s full/5s walk, repeat) makes you vomit and not feel your legs. 15 minutes of HIIT 5-times a week is a wishful thinking. It's not your typical "vigorous" activity. At my athletic very best I could at most chain 3 HIITs in a row and be destroyed for a few days.

Fair enough, I don't think it changes my the conclusion though.

On that basis, I would say that someone whose entire exercise regime is doing HIIT a few times a week for 8 minutes (24 minutes in total) is not going to be hitting the 6x multipler required for an equivalent of regular 150 minutes of exercise.

Look up Tabata intervals, used to train speed-skating Olympians.

If that is the entirety of their training regime, I will simultaneously be amazed and change my opinion.

However, I still maintain that if someone is _only_ doing 8 minutes of HIIT 3x times a week, it is not equivalent of a getting 150 minutes of regular exercise per week.

Without further context, it's impossible to comment further.

Vigorous activity is defined as something like > 75-80% max heart rate, or > 6.0 METS, not as an absolute, all out sprint. It's actually quite far from what you expect

It's better than nothing. It's better than likely some multiple of that in lower effort activities. But you probably want to round it out a little bit.

It almost certainly will improve your VO2MAX.

Maybe but if that's literally all you do then your joints are going to be wallpaper paste by the time you're middle aged