I think this is in part accurate. In the "outdoors" (outside the family, as they say) Taijiquan exists as a rather watered down diffuse style favoring specific body approach and limited application focus. They often seek to develop soft body, multi directional method (you are the center), and soft contact with "yielding" and some grappling and "push hands". Some folks would call this Taiji approach "empty center". This is quite different of course from the approach of gongfujia, however it is still offering some missing pieces that XYQ does not have; grappling, they are more for striking, softness, and global direction, they are a bit more linear (attacking center on a line). BGZ is more turning and spinning (around a center), and originally has no tuishou, nor really any great contact training regimen.
In that universe it makes sense to mix and match. In terms of gongfujia or any other original and rich example of the traditional family style, many of the elements that appear to be missing in TJQ (that XYQ and BGZ would fill in) are present if one desires to dig them out; striking curriculum, circular walk and attack, vertical power, & and the many types of jins similar to some we might see in BGZ & XYQ.
I would add the disclaimer here that while I am approaching it this way, many others cannot or won't, but regardless, it is still there, inherent in the art. But yes, different types of body method, alleged "empty center" (should be myriad or global as in gongfujia) vs vertical circle (XYQ) vs twisting/turning horizontal plane (BGZ) for the outdoor world. Gongujia contains all these vectors.