TLDR: Pop fluid node is based on PBD fluid which is partially based on XSPH with some fundamental difference, but it’s not part of the vellum solver, not yet. The Pop Fluid node contains a PBD fluid solver, which is based on the paper by Miles Macklin and Mattias Muller in 2013 from NVIDIA. The difference between traditional SPH and PBD fluid is, SPH uses a kernel to “smooth out” essential attributes around every particle, whose neighbors are within the smoothing radius, so it tends to get unstable when nearby density is inconsistent and requires lots of substeps to solve the problem, very costly. PBD fluid (which contains parts of the XSPH) tries to solve this problem by bringing the pressure solving stage(where the density problem happens) into the PBD framework, which uses constraints to solve problems (constraints avoids). Basically, a new type of constraint which is based on the positions of each particle, rather than a smoothing kernel, is introduced to help to get a more consistent density with iterations(by updating positions directly within each iteration, not accumulating pressure throughout all the substeps and then apply forces).
feel free to correct me if I am wrong