Foucault said:

The question is not “how often does he have AA or A9?” The question is how often does he have less than 9s full. If we assume he would do this with an overfull and never with less than a 9, then it's an easy fold. Even if he has the bigger hands just 6% of the time, this is a losing call because when you're right you chop a pot that is quite small relative to the amount you risk when you're wrong.

You can practically ignore the money already in the pot and ask yourself whether he's more likely to do this with A9+ or with less than a 9. If it's the former, fold. If it's the latter, call. 

Important lesson: just because the nuts is unlikely doesn't mean you can call. 

This is 100% correct.

A lot of player make the mistake of snap calling here(sometimes calling isnt a mistake but snap calling almsot alwyas is unless we have a drooler).  The bet is humongous, and you have to ask, what hands can villain actually have?  Ya up until the river shove he can have some backdoor flush draws, but VERY FEW players are shoving river with a backdoor flush, and even less are shoving river with a naked 9.  This leads him with basically boats only(I assume if he wanted to bluff he would do so not overshoving 8 times the pot, this is a spot where HE THINKS he is valueing, almsot 100%).

So how many boats do we beat?

Well we lose to AA and A9.  we chop 98, and we beat 88/96/66.  First I think we can take 66 out of his range.  Its not very likely he peeled two streets with that(although is a hand that if he did, a donk leads isnt a bad play, shoving is still lol)

So that leaves with with AA/A9 that we lose to, and 88/96 that we beat.  There are 3 combos of AA, 3 combos of a9, 1 combo of 88, and 3 combos of 96.  To sum up that is 6 combos we lose to and 4 combos we beat. 

Now it comes down to based off all previous action which is most likely.  AA is either in his range or its not, a lto fo people are 3 betting AA 100% of the time here, some people know that it can be best sometimes to just flat, esepcially if MP can be wide, or BB has a very tight image.  A9 is a standard hand to flat here, 88 is also standard to flat, 96 can be flatted especially if its suited.  Which leaves only 1 combo, which is 96ss.  So now we have 6 combos we lose to and 2 combos we beat.

its starting to look liek less of a call, even if you are able to take out all AA from his range, he still has 3 combos(a9) that we lose to and only 2 combos we beat.

This looks like a fold, without more information on villain.