HR says office location headcount is filling up...

I received an offer from a boutique firm a few days ago and had a call with HR, who told me that the headcount spots for their NYC office location are very slim now. I was confused because I was one of the earlier hires in this year's cycle (because I went directly to final round interview), and the 1R interview cycle for my school doesn't start until October. Not to sound paranoid, but is HR telling the truth or are they trying to convince me to sign ASAP? I'm also in the middle of recruiting for another firm that should wrap up early next week.