Currently tbody [1], tfoot [2], and thead [3] have "No corresponding role" for WAI-ARIA and list platform mappings directly. While ARIA doesn't have individual roles for these row groups, it does have a "rowgroup" role and similar platform mappings [4].

Furthermore, even on platforms where these elements and role are "not mapped," there is the possibility where an author has chosen to make the element focusable. Should an author do so, then the element would have to be mapped regardless of the platform. And should that occur, I would assume/hope that the mapping wouldn't depend on whether the element is a tbody/thead/tfoot or div with the rowgroup role. (Note: We still have pending to add text to the Core AAM about mapping focusable rowgroups.)

Long way of saying what I suggested in the summary: Would it make sense for the HTML AAM to just defer to the Core AAM for tbody, tfoot, and thead?

[1] https://rawgit.com/w3c/aria/master/html-aam/html-aam.html#el-tbody
[2] https://rawgit.com/w3c/aria/master/html-aam/html-aam.html#el-tfoot
[3] https://rawgit.com/w3c/aria/master/html-aam/html-aam.html#el-thead
[4] https://rawgit.com/w3c/aria/master/core-aam/core-aam.html#role-map-rowgroup