UngeheuerLich
Legend
There are indeed very few rules to run the game in the DMG or the MM. I could perfectly play 3e and 5e only with the PHB and no other book. Because the other books were not out by then.The quoted text said nothing about the DMG at all. I would have to add a passage about the DMG to begin with.
And why would they need to state that the rules for running monsters are in the book with the monsters? Natural language, remember? Additionally, I am not assuming the DMG is misleading anyone at all. The PHB does contain rules for making characters and running the game. However, you have gone a step further, assuming that therefore no other rules for the game exist. But that is blatantly fallacious. You can't even support it with evidence, because you have already conceded that optional rules exist in other books. Therefore, rules can exist in other books.
I see you have also completely abandoned any pretense of having an argument as for why your special mystical language must exist, and why it must be completely different than any other language in the game.
The DMG has guidelines to do things. Optional rules. Thise I can take or not or chose between. Or modify to suit my style of play. I can easily make up my own monsters with abilities or chose a 3pp product and play the same game.
If I change the stats of a monster I don't use a house rule. I just change the stats of the monster.