The question is, what exactly did cantabile mean back then. The art song had not yet been invented, and opera (to which we know Bach was not attracted) was only beginning to make an impact. So I am not sure Bach would necessarily mean the same as did Chopin when he used the word. Will we ever know for sure ? I guess not. Perhaps he just meant 'fluency' or something like that - I can imagine a lot of keyboard players were not exactly fluent these days and Bach felt it as his task to improve that level.
Good question, to ask what cantabile meant to Bach. If Bach meant fluency instead cantabile he would wrote it, I think. I guess he knew what he wrote. It is true that he did not like operas but does than mean that he disliked singing in general? No, I don't believe, and he has written lots for voice.
Has the meaning of cantabile changed through the age? It describes the manner to try to play as if a singer would sing, right?. Maybe the manner singer perform has changed over ages. Certain criteria about the singing style are the same, so I think. For instance I cannot imagine that singer did sing to Bach's time without a melody bow, for instance. That would be unnatural singing to me.
Maybe as first approach one could take the word "cantabile" simply with the meaning of today instead trying to look for pretty different meaning of the word like "fluency".
There is a certain point in applying dynamics and experssion in Bach even if that was not possible on the instruments of his time.
Bach liked to play on his clavichord at night, as far as I know. I don't know the exact term of that instrument, but it must be an instrument with very low volume, but the possibility to change dynamics. If Bach puts the focus on "foremost cantabile" playing for the inventions, it could well be that he had such an instrument in mind, what he liked to play himself, and what was able to produce dynamic changes.
The inventions allow for giving the both voices an own phrasing, for me it sounds the best this way, but difficult to achieve (however worthful to long for). If Bach liked to have it played "foremost cantabile", I don't see any reason to not try to play the Inventions cantabile on the piano.