Alright, so I took the lid off of it for the first time and saw that there was no airtight/tamper seal on it. (Also very slightly bigger granules of SN in places)
Not a good start but went ahead and performed the aquarium test using 2x 1 liter bottles (distilled water) and 5g of SN, here is the results that I got at the end
(Nitrite on top tab, Nitrate on bottom tab)
View attachment 122510
View attachment 122511
It looks to me like this batch has already started the oxidation process, and can no longer be used reliably. (but second opinions are always appreciated)
All in all, I can't fully say whether it could have been >99% (as was claimed on the bottle) at one time, but it definitely has some degrading going on