I for one, don't find a problem with BM or even Roger classified as a swordsman.
I find the problem in defining the WSS. Imo defining:
A. One swordsman (Roger) who is unbeatable even by WB or Garp in a fight, wouldn't necessarily be the same as defining
B. the WSS.
This is partly because:
1. there is no mention of Roger as WSS, even if in the verse Roger is like number 1 in powerlevel.
2. the logic comparison between OP world and real world.
2a. In real world, anyone who is able to use a sword in combat can be called a swordsman. Mastering other skills (for example, shooting with a gun) doesn't stop that person from being a swordsman. The logic in OP is just like this, that's why i have no problem with it.
2b. But here comes the WSS title. In the real world, what is WSS? Is it a person who can use a sword and unbeatable in any fight? No. Since a weaker swordsman, can beat the WSS, if he is more skilled or stronger than that WSS in other powers. For example, the weaker swordsman can beat the WSS by shooting the WSS with a gun. It does not make the shooter into the WSS. It just makes the shooter, the victory of the fight. This is real life logic, so i assume unless it was contradicted by Oda, the logic of WSS in OP should follow real life logic of WSS.
Poiny 2b doesn't make the WSS title meaningless. This just places that title in the proper place instead of "WSS = can beat anyone" kind of wishful thinking.
---
I hope this can provide some new perspective of being a swordsman and WSS