Then explain why Rumia talks like this happens a lot in the other pages, She must come out of this alive somehow.
...or Reimu doesn't feed on whole Rumia, leaving bare minimum for youkai to survive. I think this is about concept, that flesh is a mere manifestation, just "physical" part of Youkai and it's power. By consuming Youkai flesh, this Reimu gain her "spiritual" power. There's quite a possibility that Rumia isn't the only "dish".
Plot-twist: Rumia is letting herself get "eaten" so she can devour Reimu from the inside-out.
That wasn't serious
judging from this page and seeing Reimu distressed from some internal darkness flowing within here eyes, it could be that or something along those lines