With larger bps, popping can be trickier. If it was me, I'd tend to leave it as a male, just because it's hard to mis sex a male. Whether popping or probing, if you're getting that its a male, its a male.
However sexing females isn't 100%. They can hold their hemipenes, and if you probe and don't go in all they way, you can get false positives for a female.
That being said, if they just sexed it as a random guess or buy the size of the spurs or some other random parameter, then it could be wrong. Best bet is to find a local breeder who can show you.
Popping and probing aren't hard, but are one of those things that's best demonstrated with someone who knows what they're doing. Videos are good, but they don't beat a real live person able to help you.