When originally designed the A320 series was never intended to have a full body conformal internal fuel tank, just like the MAX was never intended to have its engines hung out in front of the wing. Sadly the "so what" of the engine decision ultimately led to catastrophe. New engine vs new fuel tank may seem to be an apples to orange comparisons but I think not as in In both cases the mod was central to the appeal of the airplane. In the MAX case no new engines meant no competitor to the A320NEO series. In the case of the 321XLR no internal conformal fuel tank means no ability to own the medium range single aisle market. Therefore the pressures to make it work are going to high.
The true test of whether the MAX debacle has really fundamentally changed aircraft certification is, IMO, how this design change is certified. Personally I have great difficulty in seeing how a comparable level of crashworthiness to an aircraft without the tank can be maintained with the current design. It will be interesting to see how this plays out as ultimately the stakes are much higher than the success of one 320 family model variant......