Most of the Zhang Sanfeng stories came about in the 17th century.
Karate did not have defined styles till starting about 1930. Geographic descriptions as Shuri, Tomari and Naha-te MA started about 1920 by Funakoshi. A general description of either "shorin" (considered "hard") or "shorei" (considered "soft") was made by Itosu in 1908. But the art was generally referred to simply as "toude jutsu," or "Ryukyu kenpo" in those early years.
(Both Funakoshi and Itosu sort of walked back their above thoughts saying they were just being descriptive, not divisional in their statements. Even in the '30's, most of the great masters had reservations of having categorized styles and had thoughts of their unification, but the War put it end to it.)
Not that there was no difference in how the art was done by different people in different towns. It was just taken for granted that each had their own way of expressing the art they often practiced together or learned from the same teachers in common.
It was basically one art done in slightly different ways. No attempt to was made to say, "If you do the art this way, it's called X, that way it's called Y, etc. till the mid 1930's.
Zhang Sanfeng from the 1300's seems to have the honor of having invented tai chi, internal/neijia CMA. Ascribing stories about him to the 1600's coincides with the trend to merge Daoist thinking into CMA during that time. I have read that
written distinction between internal and external styles did not appear till after 1800. This does not mean some CMA wasn't done "harder" and other CMA done "softer," just that it was not named "internal/external." But once something is named, the name sticks.
And once a thing is named it strives to live up to its name.
So, it could well be internal and external, rather than being two distinct divisions of CMA, were simply differences in what the teacher emphasized. One may have stressed breathing, another teacher, the physical execution. The physically stressed teaching may have been popular in the North, while the breathing emphasis was more popular in the South. Also perhaps playing a part is that Buddhism was more prevalent in the North while Daoism had a stronger root in the South.
It seems Chinese and Okinawan MA share a similar evolution in this respect, although China's was in an earlier timeframe. Sometimes, the act of naming a thing, rather than reflecting a divide, can actually encourage it. The more I learn of TMA, the less stock I put in names. At its root, all styles are MA.