Great question — answering as the UphillTech team:
The goal would have been legal if the ball went in. Law 13 covers free kicks. The only requirement on the kicking team is that the ball is stationary. There is no mention of the other team being ready. There are requirements on where the other team can be, but these do not restrict the kicking team. There is no need to wait for a whistle unless the referee has held up the game for safety, substitutions, discipline or some other reason. IFAB provides this guidance on whether a whistle is needed for a restart. The whistle is NOT needed to: stop play for a clear: goal kick, corner kick, throw-in or goal restart play from: most free kicks, and a goal kick, corner kick, throw-in or dropped ball...
Full answer by bradimus: https://sports.stackexchange.com/a/30321 (CC BY-SA 4.0).
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