I think the limit "boundary-stone" idea and the limit "end/purpose" idea are not as far apart as might seem.
Lucretius does use the exact phrase "deep-set boundary stone" (alte terminus haerens, I think) in Book I.
There's something to all of this, but I haven't been able to crack it. I've written here before about the English and Colonial practice of Beating the Bounds. The ritual is thought to have had a Roman origin.
So a boundary stone is a definer of limits; but it is also (or was) the subject of ceremony and ritual, a focal point of collective memory, something agreed upon and quite literally "settled"...
I don't know. It's uncanny how often the words 'borders' and 'boundaries' and 'limits' come up in Lucretius. But I don't have a satisfactory resolution.