The readme.txt file check is not specific enough
The readme.txt file check is intended to check if there is a readme.txt file that has information about the theme, but it will look for any readme.txt file.
If the themes readme.txt file is missing, but there is a third-party readme.txt file for a library inside a subfolder, the check passes.
Example theme: https://themes.trac.wordpress.org/ticket/102187 The readme.txt file is inside https://themes.svn.wordpress.org/momentog/1.0/genericons/
We can check if the readme.txt file is in the theme root directory. And we can apply if the validator checks pass or not? https://wordpress.org/plugins/developers/readme-validator/
If there is a README.md file, it's not necessary to be readme.txt? This theme doesn't contain readme.txt, https://themes.svn.wordpress.org/games-online/1.1/
It depends on what format is decided on once the readme is parsed in the theme directory. https://meta.trac.wordpress.org/ticket/215#comment:41
I saw that the plugin directory checks for both but uses readme.txt if that exists. So I think it should be the same for themes.