Found there's no need to ever tap the screen :) Instead, it's all doable by pressing your finger on the screen and swiping back and forth. This means you rarely miss blowing aliens up! Fun enough, but little to no challenge for it.
Sorry for the late reply. I believe I added that ability in as a hidden feature so that if people found it they can use that as a technique. However, I think I removed it in most cases, I will double check and see if I have missed anything. If I did remove everything and its still in then, I think it might have something to do with Unity and its input system. If that's the case then I will look into it further.
I have had a look at the project, at the moment there seems to be nothing that I can do to fix this so in the meantime it will stay in but I will try to look into this more and see if I can figure this out.