Well, it seems like one of those information is outdated because they are contradicting :D. However, I checked and TestFlight purchases on macOS are not connected with my sandbox account, so I guess the note in the App Store is newer. I think in the past this account was used for TF purchases but it's no longer the case.
I checked it purchasing a subscription when my sandbox account was set and the purchase did not appear in the App Store Connect next to my sandbox user.