The general idea with this ThemeCheck service is cool & useful. I like it, and despite some reservations, will continue investigating.

Certain of the implementation-details that have been selected, look like they are or invite 'issues'. Certain of these are readily addressed/fixed, while others; maybe not-so-easy.

Calling what is done here "validation", and then asserting that "security and code-quality" objectives have been achieved, is overstating what it is & does. This terminology & claim poses the service as something more-authoritative than it is. "Validation" of code is a term that already has an established meaning; it's an advanced & challenging ('impressive') field of computer science. It's virtually impossible than any 'real' validation is being done.

So what is the ThemeCheck website doing, then, if not "validation"? Well, this service is of course based on a top-ranked WordPress plugin called Theme-Check, written by Simon Prosser and Samuel Wood (Otto). On their Theme-Check FAQ page, they describe in more detail what it does:

... The theme check is designed to be a non-perfect way to test for compliance with the Theme Review guidelines. Not all themes must adhere to these guidelines. The purpose of the checking tool is to ensure that themes uploaded to the central WordPress.org theme repository meet the latest standards of WordPress themes...
...
This theme checker is not perfect, and never will be. It is only a tool to help theme authors, or anybody else who wants to make their theme more capable. [emph. added]

That's a very humble description, compared to ThemeCheck's terminology & claims. Toning down their highfalutin language shouldn't be too hard. (I would check with Mr. Prosser & Mr. Wood, and ask to quote them & their FAQ-language. Those are heavy-hitters at WP, and their 'implied endorsement' is meaningful & valuable. )

Next: the numerical scoring system. This is where the overall working-design of the ThemeCheck website might be harder to tweak.

When we say things about products in the market, or more pointedly, when we publish statements about things & people, we fall under moral & legal obligations. We can't just up & trash-talk the products of other entities, without responsibilities & consequences.

If we are going say negative things about products, we have to have our ducks in order. It has to be real & verifiable, if we are going to eg rate the Ford Super Duty at 0, and the Dodge Cummins Ram at 100. Ford could take a dim view of that, and they do have recourse.

To the extent that this scoring system is arbitrary & subjective - that numeric valuations have been 'pulled outa thin air' and assigned to points of Guidelines deviation & compliance - the ThemeCheck website is on shaky ground. They're 'makin it up' - and can be challenged, successfully.

Somebody is likely to 'take exception' to how their theme is be posed, at ThemeCheck. They have the right to ask that an accounting be forthcoming. People who publish claims & characterization about products are, after all, held to be morally & legally accountable.

It would be a big surprise, if this scoring system is a consistently accurate or meaningful reflection of the usability & merits of themes. Best to tackle that weakness, sooner rather later.

Other than than ... nice idea, and best of luck with it!