Would 8 minute HIIT a few times a week do the job?
It almost certainly will improve your VO2MAX.
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.
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.
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.