[Cushman]

Naturally, there is no such objective principle for me to articulate. Everyone speaks according to the language center of their own brain. Some ways of speech are more different than others; some are so different that communication is hard without speaking slowly, and some still are so different that communication is impossible and you must resort to pointing (and even pointing is not universally meaningful). At some point along the line we call it a different language, but it's not representative of any sharp distinction that exists in reality.

Asking a linguist "are these the same language" is as useful as asking a biologist "are these the same species"; it all depends on what you want to use them for.

(Although I don't particularly see the relevance of that to whether or not leet has a substantially distinct grammar from English, of which I remain unconvinced.)