not experienced with pools, but built plenty of houses, decks, sheds, etc...
You are always better off digging down, than building up- but building up can be done
You have to take measures to ensure what you build up doesn't erode, or wash away
And you have to pack the new stuff in- your existing ground has had rain and whatever compacting it for years, new fill will compact and leave "holes" in your ground under a pool liner...
So, if they landscape well, and pack the fill, it can be done- but digging out is easier in the long run IMO...
Good Luck!
Madwil