YAML is a data serialization format. Like JSON.
As a user of ELPA's yaml-mode, I was surprised too it isn't coherent with lisp-data-mode, js-json-mode, json-ts-mode who all inherit from prog-mode. And I had to disable the input method I usually enable in text mode buffers, re-enable flymake and my shortcut for flymake-show-diagnostics-buffer...
Since yaml-ts-mode is new it'd be a good thing to be coherent.
But thinking twice is there any concrete gains of inheriting from prog-mode rather than text-mode for the end-user?
Don't we already face that dilemna with nxml since XML is used to serialize data?