by Jokemeister
I guess the answer is simply that the mechanics and art of the game are done by different people and for different purposes and possibly even at different times. The producer probably (possibly?) never thought about making sure that both worked together or maybe it did cross their mind but by the time they realized it was an issue, maybe it was too late to change (eg the art and mini had already been completed and they didn't want the additional cost of commissioning new art or sculpting a new mini).From an art point of view, the art would have been commissioned from an artist. It's entirely possible that the artist doesn't play D&D or have any familiarity with the Ravenloft universe or simply relied on a brief from WotC for the art. That brief would possibly have included the character backstory and a brief around the "feel" that WotC wanted to capture in the art (ie they may not have known that Strahd was mechanically a magic user and mechanically unable to wield swords). Put all that together and you come up with character art showing Strahd wielding a sword.
Similar story with the actual miniature. The original sculptor was likely basing the sculpt on art that he received. If that art had the sword, then the sculpt also had the sword. In terms of the boardgame, I believe the minis were all based on old sculpts (rather than new sculpts specifically for the board game) although I could be wrong. Either way, you can see how the sword crept into the art and miniature sculpt from the background of the character itself.
However, in terms of in-game mechanics, D&D traditionally has very strict rules around what magic-users can wield. When Castle Ravenloft first came out, magic users weren't allowed to wear armour or use swords so the in-game mechanics for Strahd - being a magic user - wouldn't have any kind of sword attack.