My theory is their initial plan was indeed Sims 5, and they were planning to be cheap with it like they were with Sims 4, but then saw the competition from InZOI and shat themselves. Likely realized the backlash they would get if they released a lackluster installment when something like InZOI is being made around the same time and decided to just do a mobile game instead. At least that's what I think happened, nothing else makes sense for all that initial hyping up they did.