It seems all to often that this is the case with "custom" AR rifle "builds"...they all seem to say 4-6 or 6-8 weeks and in reality it ends up being 6-8 months or longer! The problem is, if they tell each potential customer that inquires that it will be actually 6-8 months, most customers are going to move on and look elsewhere...so they tell you 6-8 weeks for you to go ahead and "order"...then you basically have to wait until they finally finish it. I don't like this strategy, but it seems to be the norm nowadays.
On the other hand, most custom AR "assemblers"(not builders) outsource all the parts...so they are sometimes waiting on barrels, uppers, lowers and such.
I had a friend go thru your exact same issue...told 6-8 weeks...just kept extending the deadline because of "waiting on barrels", "waiting on coating", waiting on basically everything...he finally got it in about 6 months. Unfortunately it did not live up to the "assemblers" claims nor did it live up to his expectations.
Be patient is all I can say and keep your fingers crossed that it lives up to your expectations once you finally do get your rifle...