This is simply not a valid test, at all.
There’s no conclusive observation that can be made by looking at one specific aircraft flight model configuration and then applying that conclusion to the entire flight simulation. How well a particular aircraft meets book values is entirely dependent on how well the flight model author adjusted the values to make the book values possible.
This is exactly the same in both MSFS and X-Plane. X-Plane only uses geometry to the same extent MSFS does, for the most part. All the complex study level flight models developed in XP heavily use datarefs to adjust various tables and scalars to modulate the output of the simulation, because all simulations are imperfect.
If the flight model designer has not input the correct parameters into the model, then you get a crappy simulation, both in MSFS and XP. It’s why the default 172 in XP flies like it has no idea what longitudinal stability is, while payware offerings are much better: that doesn’t mean XPs flight model overall is garbage, just that the configuration of it may be for a given airplane. Similarly, taking the default 787 which doesn’t match book and claiming it means something about the core of the MSFS flight engine is just misguided.
In the right hands, the MSFS modern engine is going to produce some seriously accurate aircraft. How do I know that? Because our Working Title CJ4 does actually hit those book values at all regimes, with correct N1s, fuel flow, climb rates, over various altitudes and ambient pressures. Not only that but we have stall speeds within a knot of two of book, proper approach angles, correct bank rates, etc.
Is the MSFS simulation completely perfect and without limitations or quirks? No, but neither is XPs, by a long shot. These strange questions and tests are apples and parsnips.
-Matt