I don't believe you can get a strong bond gluing the molded figure base (of any material) to the decorated base, period. I've tried different glues, and they pretty much all bonded very poorly with the detritus on the base, popping off quite easily.
Looking at the website's photos, it appears that he attached the figures first, then decorated the bases; either that, or he flocked the molded figure base after attaching the figures to their bases.
I have found that I can only make it work if I attach the figure to the base material, first, then flock the base. It is the only way I can get a strong bond between figure and base.
Personally, I use Hot Glue or E6000 Glue for the job of attaching the figure to the base. These two glues work well enough for: metal, resin, 3D printed figures of any material, HDPE/LDPE figures, as well as vinyl figures. I switched to only using MDF basing materials around 15-20 years ago.
I attach my base flocking (mixtures of different colored sands) to the MDF bases using PVA Glue. The best PVA Glue to use is a bookbinder version: dries crystal clear; remains tacky for 9+ months, and counting, so the sand will not easily fall off; has a more-than-enough strong bond.
Note, also, in the website's photos that the author places one, or more, tufts of grass material, between the figures' legs on their molded bases, to hide them. That is a style choice, but to me, it would be overdone if it was present on each and every figure! My armies are typically 20-50 figures each, so it would be too much, too redundant, for me. YMMV.
Here is my first example of what I typically do (Caesar 1/72 scale Dwarves). For these Cyclopes figures, I added Hot Glue texturing, prior to the PVA Glue + sand mixture, to further hide their molded bases which I think worked exceptionally well: Dark Alliance 1/72 scale Cyclops, injection molded plastic figures, with thicker molded bases. Cheers!
