For all you know, the guy could’ve thought it must be her but wasn’t sure, and showed her her own plan to her see if she says “oh that’s me” or something.
Molly’s tweet itself didn’t necessarily seem presumptive of sexism but her “didn’t have the heart” comment seems a bit arrogant as she’s assuming that he’d be embarrassed at not recognizing a celebrity he idolized, but she’s not exactly a celebrity for the general public to recognize her and the guy may not even be a particular fan of her, just a running stats nerd.
I’m having a hard time imagining this guy wouldn’t be doling out similar gratuitous advice to a guy next to him telling him it’s his third marathon that he ran under his PB.