I have to say something, there is a pecking order of accuracy:

1. Museum, hard to argue with the original object, you can mis-understand though.
2. Book, researchers do on occasion visit museums but don't have a clue what they see.
3. Re-enactors, who read books.
4. The SCA, read the books but are not so interested in accuracy.
5. Hollywood, don't read the books.

and at the very bottom,

6. Renfairs who get what they know from Hollywood, completely inaccurate is better than most fair goers dress.

As far as "plaid-like patterns" go the origins are lost in the sands of time. They may predate the weaving of cloth as the weaving of grasses into baskets is a precursor technology to weaving cloth. Dating plaid-like cloth to the Renaissance is off by five thousand years maybe even ten thousand, we don't know. What little we do know is that the Celts had striped clothing in 500BC and if you have the tech to do stripes, you can do checks at will. We haven't any hard data on what the Picts wore and the Scoti were (likely) dressed like the rest of the Irish in stripes. What is funny is the word "varicoloured" used to describe Scot's dress, what the hell does that mean, tie-dyed?

I realize I am preaching to the choir here but, I have a reason here, every well thought out Internet Site that I go to proclaims loudly that the Re-enactor should avoid clan tartans as being "out of period" but we do not know that. What we know is that none of the new Clan Tartans are period, but the old ones might or might not be. The fact is we suspect the whole Clan Tartan thing is pure BS but we do not know. We just don't have any data one way or the other, just romantic rumors.

So for the most part the claim that Clan Tartans are not period is just as baseless as the claim that they are period. We don't know.