At which point does a mess of code become a "game"? There are no standards simply because there can be no standards for something as loosely defined as an "alpha", or even more generally, a "game in development". You can pick a title and hold it to a standard of your choosing, sure, but you can't generalize a set of standards for these, as each developer sets their own. In addition, each genre would have its own on top of that. A non-interactive series of pictures can serve as a sufficient gameplay representation of a visual novel for one developer, and another may consider a few plain text files the same, whereas a more self-conscious company will only release full, compiled segments for others' perusal - and they would all be correct, in their own ways.
For instance, since I am a programmer making a few programming projects of my own (though none so important as to be mentionable), I can work with an example. My applications usually start from the face - I begin by taking a form window and liberally sprinkling it with buttons, text fields, checkboxes, and all the other stuff that goes into the program's interface, in accordance with what features I intend the program to later have. Before I compile the executable with just this one nonfunctional form full of buttons, I do not have anything at all - unless you would classify "A thing must exist" as a standard, I have nothing to release. Once I compile the executable, I have something. That something represents a measure of work I have done towards the creation of the final program. It is not functional, it is barely representative of the final work at all (and I will likely remove some buttons and add others as my idea of how the thing must work changes), and yet it is there. Now, I would not make such a thing public and claim it to be a "release", or an "alpha" - because such are my standards, general, applied to all of my work. I would post a picture of it maybe, and keep working. But that doesn't mean that somebody else - somebody who has less personal standards - couldn't actually call it a release, and be justified in doing so. Because like it or not, it is a development snapshot, representing progress towards the final program that could be presented for evaluation - and the magnitude of said progress and the fact that its entirety could be summed up with an image, does nothing to counter that fact.
In other words, standards in regards to things labelled as "in development" are entirely personal. Whether or not you consider a given thing being "worth" the name of "alpha release", and whether or not you think it could be worth paying money for - be you a potential customer or a developer - it only speaks of your own standards. If the "release" is just a main menu leading to a loading screen, or just a main menu, or just a black screen - as long as it is anything at all relevant to the progress of development, it qualifies. From there it's just a matter of how much the developers care about whether or not anybody else tries to play their game at a given moment in time, and whether or not they care for any feedback at all.