We know that quarantined subs don't show any sort of sub count so using tools like https://redditmetrics.com/ could only give us the pre-quarantine numbers.

So perhaps this is just a matter of personal judgements.

What are your opinions on the validity of the sub count?

Personally I have my doubts, if it did have over a million subs, the it would show it through its upvote count and user activity.