The "scared of manjuu" joke, which semi-famously also appears in Imperishable Night's extra stage, originates from a very old comedy skit of unclear origin where a man convinces his companions that he's scared of manjuu so that they'd try to "scare him" by throwing a bunch at him, which he then happily eats.
In this case, it's kind of a general reference to reverse psychology.
She might lie to people that she's scared of manjuu, but...
That's right. She actually likes manjuu just the same as us.Huh? Shinmyoumaru!?
(Marisa)Tell Reimu that I took the Mallet.I need to make things clear with Seija....Wanna take this with you?Marisa... It's just an expression.'Tis a damned crime how this land only ever brings
the very worst matches together...Seija won't eat just any kind of trash.She'll keep force-feeding her manjuu until she gets heartburn and spits out the truth.
But... it might take a little trickery.