"or so we like to pretend" refers to what has been said before (the farm is superfluous) and questions it (is it really superfluous, after having existed for four centuries?) It basically means: "We like to pretend that the farm suddenly became superfluous in 2010, but is it really?"

“Or so we like to pretend” here is being used to indicate that the previous part of the sentence may be untrue or not entirely accurate. “In 2010, it is suddenly superfluous, or so we like to pretend” means in this case that Tuttle farm is probably not suddenly superfluous in 2010. This usage is commonly found in the context of sentences such as “or so it would seem,” which means that things are not entirely as they seem.

"or so" here could also read "as such" It refers in this sentence to the pretending that the Tuttle farm was superfluous in 2010.

Be aware that "or so" can have a different meaning if it is at the end of a sentence. eg. Question: How many eggs did you eat? Answer: I'm not sure, about 10 or so. In this context it means "approximately"