I mean he is practically a pseudo legend. 6oo is way to low for a legendary Pokemon like him.
No, Zygarde is a legendary Pokemon. It doesn’t fit the criteria for a pseudo legendary Pokemon, since it does not evolve (all pseudo legendary Pokemon evolve twice).
You are also wrong saying that 600 BST is low for legendary Pokemon; all the legendary Pokemon you receive from events have base stat totals of 600, and there are several others with 600 as well. There are even some with below, like the legendary bird trio which all have a total of 580. Take a look at this statistical average for legendary Pokemon; Zygarde is pretty average to me.