The leading theory seems to be that it's meaningless gibberish, in part because asserting that it isn't requires some degree of specificity as to what it could be, and there's little agreement on that aspect. Which is to say, it's the plurality theory but far from the majority theory.

It's also known that the text makes no sense as an encipherment known in the 17th century of any known natural language. Statistical analysis of features suggests it's not random gibberish... but also distinguish it from natural language. It's also prone to a high degree of repetition or near-repetition (think phrases like "burgle bugle bugie", with one-letter changes between successive words).

Personally, I think it's gibberish that was constructed to have some degree of plausibility for a would-be cryptanalyst. Or maybe even something as prosaically simple as calligraphy practice, given the unnaturally repetitive nature of a lot of the text.