I expect there were more than one pair of plaid pants on campus then. Why couldn’t Northam have had a similar pair?
Occam’s Razor. His close friend owned such a pair and was photographed wearing them. There is no photographic evidence that Northam ever had such a pair.
Many things are “possible”. Fortunately that’s not how our rules of evidence work.
Northam could have done anything, unless it could be proven to be impossible.
And of course that same standard could be applied to any of us, meaning that our sole defense is reduced to proving that a charge made against us is impossible to have occurred.