I can TOTALLY get the reasoning behind supporting only a single engine but the reasoning they write there are either wrong or misguiding.
It is 100% legit to give that up, it is 100% legit to go with the clear winner in your mind (it is the same in my mind too), but the technical reasoning is weak IMHO.
if you wanted to support both, you can, but you clearly don't (and that's ok too).
It's great that the simplicity argument doesn't really apply. It's a testament to developers getting better at programming in the large over the years. The database layer can be neatly abstracted away from the business rules.