Though if you look at the NoLimit City report system (not the one on the slot screen after the bonus win) buy feature winnings multiplier is calculated by winnings /cost of buy feature.
NoLimit City doesn't report it by winnings / cost of buy feature. It's winning / bet amount. Proof below, you can look at my buys on this slot, these are at .10c bet amounts
Here's the replay of my 425x
https://replay.nolimitcity.com/show/kirureoberu?device=desktop&language=en
He is not referring to that report/record imho, but he is referring to the bet history. You can try to check your bet history on that No Limit City slot. The bet history will show you how much you spend for the bet, so if you use buy feature then it will show that pay the amount for the buy feature. https://prnt.sc/w7l84l
This screenshot shows the difference between normal regular bet and one with buy feature. But indeed if you look at your best record bet then it will always show you your base bet amount no matter you use buy feature or not.