If the engineers that originally implemented the function intentionally chose the slower version, a quick comment as to why would have prevented this from happening in the first place.
One of the first things you're taught when learning pytorch is that you're not coding in python, but actually creating a network graph that is loaded and executed on a GPU. Other common sense things is knowing that you shouldn't use stuff that is in the stdlib or in numpy and use torch.* variants instead, not doing so will incur either undefined behavior, cause massive memory copies between the CPU and GPU or most likely, error out at runtime.
Note that this is a repo that is forked from the official repo, it's a community repo focused on inference and thus doesn't care about training so it has completely different considerations than the original code.
As everyone said, this is more performant because x is being modified in place, the reason this was not done in place is because you can't train a neural network if an instruction is being done in place. During training a network goes literally through all operations that were done and see how well they performed so they can be adjusted using a secondary value called a gradient, this is done during the backwards pass. If you replace something in place you're essentially overwriting the input values that were passed to that function, and by extension, the output values of the function called before, essentially breaking the network chain, unless you also copy the inputs together with the gradients, which would cause an even worse performance hit and be a memory hog.
The breakage bug later in the issue is proof of this, when sampling to generate an image only the forward pass is done on the network, but textual inversion requires you to train the network and therefore do the backwards pass, triggering the error since the dependency graph is broken. I should also note that technically the add operation should be safe to do in place as it's reversible, but I'm not a pytorch expert so I'm not sure exactly what's going on in there.