Add in the fact that they had planned on buying his app outright told him they would hire him instead to speed up the process, and it's an especially crummy thing to do.
It avoids the possibility of obsessive types getting agitated over an explicit rejection, et c. Anyone clever will see ghosting for what it plainly is: an explicit rejection.
It’s just business, not some trusted friend ghosting you on lunch plans and not calling. This is how business works, and it’s not rude, it’s just the protocol.