Is he the same expert who made such a professional job of servicing your pistol? There are experts and those who think they are! He's mostly wrong, is the correct answer.
Pellet matching is more critical in Match air rifles due to the 10 being 0.5 mm across, thats 40 thou in old money! But even in a pistol there is a dramatic difference in pellet performance, but even with a miss match most good quality match pellets will still hold the ten, so stick to RWS R10 or H&N, but the cheap brands can be quite haphazard. For what they cost per shot, parts of pennies, its not worth using cheapo's.
Good Shooting
Robin
Walther KK500 Alutec expert special - Barnard .223 "wilde" in a Walther KK500 Alutec stock, mmm...tasty!! - Keppeler 6 mmBR with Walther grip and wood! I may be a Walther-phile?