Documentation states that siteaccess has priority over group such as https://doc.ezplatform.com/en/master/guide/siteaccess/#scope

This is not the case. It looks like the siteaccess settings are just merged.

For example:

Setting languages in site_group to [eng-GB, fre-FR] then setting languages in site to [eng-GB] the config resolver will return [eng-GB, fre-FR] instead of the expected [eng-GB].