That's not the correct takeaway from that question, though I can see how that mistake can be made. A monumentum can indeed mean "monument" in our sense. You can see this plainly spelled out, too, in the dictionary.
I. that which preserves the remembrance of any thing, a memorial, a monument; esp. of buildings, statues, galleries, tombs erected to perpetuate the remembrance of a person or thing
The primary definition isn't a grave, but rather some sort of marker for remembering someone or something, including a massacre.
It can also be used more metaphorically, such as when Horace says "[he has] created a monument more lasting the bronze" (exegi monumentum aere perennius, Odes 3.30.1). He is talking about poetry, not a tomb here.
While it can often mean a tomb, it's certainly not restricted to only that one definition.