If he went undrafted that means any team could have invited him to camp or signed him, thus they all passed on him.
He was actually signed by the Warriors after Summer League in the year he went undrafted, with other offers coming from the Lakers, Mavericks, and a reported "unnamed Eastern Conference team", which for all we know could have been the Celtics. And more teams were supposedly interested. Lin just went to the team that had little depth at guard and was willing to offer some partially guaranteed money.
Lin was at the level where he was clearly going to get an invite to training camp somewhere, if only because some teams would try to chase an Asian-American fanbase, so he was probably going to have some options. Lin was probably the most hyped undrafted free agent straight out of college that I have ever seen.