As Kenshi said, Yamcha was good enough to be in the top 8, a huge deal at the time, and was likely seen during the prelims by that competitor. Goku also said he could barely see Yamcha's Rogafufu-ken when they first thought and he was capable of beating one of the Orinji Temple bullies that was stronger than Kuririn initially was, so it's safe to say his performance there was enough to shock the more normal competitors with his strength and speed.