This is exactly what the rules tell us.
Why otherwise go to the trouble of making sure you can't combine Flurry and Monk Weapons...?
Why go through the trouble of making the Hand Crossbow a Light weapon when the Light property only has an effect on melee weapons? Why Why go through the trouble of making abilities like a Halfling's Naturally Stealthy, a Wood Elf's Mask of the Wild, and an ability of the Ranger literally named Hide in Plain Sight when the rules for stealth say, "You can't hide when someone knows where you are"? Why does Sneak Attack and progress without occupying the class ability slot, while other abilities like Indomitable, Brutal Critical, and Destroy Undead do? The rules are full of uneven designs and even outright design errors. Assuming that every rule is of equal quality or that what a rule tells you about design is both intentional and correct is foolish. My statement wasn't, "Is this really what the rule says?" it was, "This rule seems of dubious value and is probably safe to ignore."
Yeah, sure, if the Monk has a Sun Blade at level 4, it's kinda broken, but is it that corner case so broken and so common that you can't trust the DM? You have to similarly punish a level 11 Monk with a
+2 dagger? Is it worth the rules burden of the extra condition?
Another example: Sage Advice has said that Crossbow Expert doesn't allow you to dual wield hand crossbows. Does that rule meaningfully reduce player damage? No. Will some players find it fun to go medieval and John Woo at the same time? Sure. Is this a restriction that's worth paying attention to? Probably not.
Another example: Martial artists using two weapons is pretty iconic of the genre. Is it broken to allow a Monk to use two Monk weapons at once without two weapon penalties? That seems unlikely, since a Monk can already do the same damage with just one weapon. Is it worth it to prevent your players from trying to do that? Absolutely not. The rule that detracts from the game by limiting player choice without limiting player power.