It would be one thing if Pennywise were quoting Richie’s friends when he said it, taunting the kid with his intimate knowledge of him and his friends, but as previously established, it’s not a line that was said by any character either prior or post the ghoulish reading from Bill Skarsgard.
Why would Pennywise say “Beep beep, Richie” after jumping out at the bespectacled child in a room full of clown dolls?
I’ve been thinking on this for more than two years now, and watched the movie many, many times, and I’m still nowhere near a totally reasonable explanation.