No, there's no way to tell—the paleontologists probably just took a bunch of bones from the site and threw them together in a way that looked cool. /s

This video [0] tries to answer the question of how we know what we know for a different species but I imagine the methods are similar.

[0] https://www.youtube.com/watch?v=5vea06e6x_E

If they could eat from higher branches than others, they could avoid competition for resources.

That’s an ongoing debate within paleontology that often takes place on a species by species basis. The argument goes that since diplodocids (which this species is not) had heavy muscular tails, their center of gravity was near their hips, making it easy to rear up since they were effectively already a balanced seesaw and could use the tail as a third point of contact to balance. Species in Mamenchisaurus share similar pelvic and tail features and M. youngi was show to have a stiff neck that couldn’t lift very easily, so it’s inferred that these may have reared as well. There are center of mass and skeletal models and stuff to determine whether rearing is possible but one hasn’t been made for this species specifically.

Sidenote: you underestimate the cardiovascular cost of pumping blood up a 5-15 meter neck. It’s not at all clear that a rearing strategy is more expensive energy wise. In their case it’s less spending energy to standup than just leaning back to let their skeletal structure and center of mass do the work.

Mostly I think this pose is a matter of logistics. They probably just had more vertical space than horizontal to work with for this exhibit. Even though they’re fiberglass, the casts for these guys run well into the tons per skeleton so it can be challenging to mount the armatures in an existing structure and it turns into a game of fossil tetris balanced by the cost of structural support modifications needed (there almost always are for a fossil of this size).

meh just put the head on the end of the tail, a lot less effort and no one will be any the wiser.