I try to provide more details for my above answer.
In fact, this question is a dilemma problem.
On one hand, the Diophantine Equation Problem (DEP) is undecidable according to Matiyesevich’s theorem (Matiyesevich’s theorem answers Hilbert's tenth problem, and Turing’s Halting problem answers the generalization of Hilbert's tenth problem, that is, the Entscheidungsproblem); but on the other hand, there isn't any undecidable problem in NP according to the definition of NP (decidable and verifiable).
That is to say, either DEP is not in NP, or DEP is in NP. Both of them concern the definition of NP.
If DEP is not in NP, that means problems in NP (NDTM＝NonDeterminstic Turing Machine) are decidable and verifiable, that is to say we accept P=NP (NDTM).
If DEP is in NP, then NP (NTM＝Non Turing Machine) contains decidable and undecidable, obviously decidable is verifiable, therefore the problem is whether undecidable is verifiable? In fact, that is the famous problem of P vs. NP. Certainly, undecidable is unverifiable, so NP corresponds to NTM (Non Turing Machine) instead of NDTM (NonDeterminstic Turing Machine).
Going on the premise of DEP is in NP (NTM), we think that the NP (NTM) is Nondeterministic Problem (undecidable), and the current definition of NP (NDTM, decidable and verifiable) has lost this nondeterminism (undecidable), so we think that it needs to be questioned.