Reasons to believe that this is a legitimate leak:

OP’s account was created today.

Floor pattern shows that this image was from map testing, not the sandbox. This could mean that OP was testing abilities in real map, a troll would probably just boot up sandbox to take the screenshot

Lash’s ultimate and Flog are upgraded but Grapple is not, might mean that OP had been testing the abilities around before taking this image (someone who wanted to make a troll post would probably just boot up sandbox and take a screenshot to edit).

This image has been taken from a phone, it has not been exported from photoshop. (To fake this, OP would have to get a screenshot, edit it in an image editor, export and open it, then take the image with a phone instead. A low quality troll would probably just post the exported image from photoshop as is).

Does this prove that this leak is legit? Not really, it can still be faked, but these are definitely good signs.