Joseph,
If it’s a clear cut halacha why did the MB need to include the scenario? He could have just written, “You need to wear a hat to daven.”
Instead, he wrote “in our days we meet royalty with a hat, so too, we wear a hat to daven.”
That means that in our days where we specifically DON’T wear a hat, you don’t need to (or even, shouldn’t, if you want) wear a hat to daven.
Furthermore, I do agree that for kiruv sometimes we put somethings aside to maintain the more important matters. means that it’s not halacha because you don’t just change halacha for kiruv purposes.