I'll give you the fuller quote about the Quaker "thee" and you can decide for yourselves.

... It is also used as nominative case (a use unaccounted for) by Quakers (Perhaps thee has noticed the point in our Friends journal ...)

So he acknowledges that the usage is unusual. I would be suprised if the quotation were a mistake. There were, presumably, other examples.

Perhaps Paulb can enlighten us.