Meng Qi had never seen Gu Xiaosang's handwriting, so it was evidently impossible for him to judge the identity of the writer just based on the handwriting. But considering that he had discovered Old Zhong's whereabouts through her yesterday and the fact that he received definite information today, it was hard not to make the connection.
The simplest, most direct, and logical rendition of events would be Gu Xiaosang following Old Zhong after finding him. When she found his hiding place, she considered the possibility of an ambush or trap as well as the limited influence the Luo Denomination had here. Thus, she intentionally notified him so the righteous factions would brave the unknown in her place.