I haven't used Signal to know exactly how it works, but if its like some other chat platforms, its kinda similar to email. if you have chatted with them before and go to type a name into a new chat, it will autofill.
its also possible that Goldberg had been the VN equivalent of "@"d previously, and that pinged him into the same autofill. pretty easy mistake to go from talking about someone, to go to "@"ing them instead which likely pings them.
least likely is Signal works like VN where you can @ anyone, regardless of you talking to them before or not.