This is just a matter of "game mechanics", the wiki is talking about gameplay, where some areas are considered part of the MT area. Tianheng, but they are not part of MT. Tianheng in lore.
Where exactly is this said? I don't remember it being said that he uprooted and cut through all the mountains of Mondstadt or that he reached the edges of Liyue.
Mount Tianheng is a Mountain Range, not a Mountain. I didn't include flat regions only the Mountainous Regions. Also, if you read Lithic/Lithos? Spear, it kinda implues that Liyue Harbour's Bedrock might not be natural at all and it waa created by Morax. Tho, I didn't include it.
Primarily by exploring the map. Might not be the most acceptable answer however you can see a lot of stuff that are not written in book or skill when you explore the map and compare Geography.
When you look at the Dragonspine's and its surrounding Geography by gliding from The Skyfrost Nail or even the World Map, you notice weirdly flat Mountain Peaks. The lands surrounding Dragonspine seems incomplete and unnatural. Weirdly jutting put at angles.
In Lore explaination,
According to Venti, the archipelago used to be called the Haar Islands. The islands were created during the terraforming of Mondstadt; as Barbatos reshaped the region, he split off the tops from mountains which then landed upside-down in the sea, becoming the archipelago. Which when looking at tge World Map in Dragonspine we can see flat top Mountains. Moreover,
Mona : I once read somewhere that the Anemo Archon Barbatos, once used his divine power to mobilize the winds and blow the ice and snow from the face of the earth. The whole landscape of Mondstadt was changed.
Mona: The mountains of that age were replaced by the vast stretch of ocean we see today. Still, a mountain that high? Drop it into the ocean and it'll still leave a trace.
Mona: The summit still reaches just above the water's surface. So, the place known as Pilos Peak in Leonard's day is today known...
Mona: The summit still reaches just above the water's surface. So, the place known as Pilos Peak in Leonard's day is today known...as "Musk Reef." (Unreconciled Stars Event)
But yes, the average depth of the Ocean is 3700m.