Under state contract law which looks to the intent of the parties, explicit statement in the arbitration agreement that it was to be signed prior to being given effect or modified meant that omission of the drafters signature meant that there was no contract.
http://www.ca5.uscourts.gov/opinions/pub/17/17-50341-CV0.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.