Here's my take (which I'm quite fond of)
Page 85 of the core book tells us that magic resistance keeps magic away from the protected creature.
A sword that has been opened for enchantment but does not have any active magic on it is not magical by any reading of the situation that I find reasonable
ergo it isn't resisted.
Also, I googled for "enchanted sword" and "parma" from this site and here is what I found on the first page I'm sure there's lots of stuff here to entertain and inform, I wouldn't be surprised to find a similar list on google's second page, we've been here before.
2006:
https://forum.atlas-games.com/t/enchanted-swords-and-parma-magica/259/1
https://forum.atlas-games.com/t/parma-that-favorite-topic/277/1
2007:
viewtopic.php?f=4&t=2995&start=0
2009:
https://forum.atlas-games.com/t/swords-wards-and-penetration-again-and-again/3625/11
https://forum.atlas-games.com/t/yet-another-parma-magica-question/3622/1
2011:
https://forum.atlas-games.com/t/magic-sword-vs-faerie-might/5529/1
2015:
https://forum.atlas-games.com/t/enchanted-swords-from-the-confraternity-of-roland/10162/1
(Just passing through, I've still not got my life to a point where I can regularly return to this forum (although finishing Andreva -the magical sword maga- has coincidentally been a metaphorical itch that I've been meaning to scratch for over a year.))