Did Rock Lee learn the Leaf Dragon God? It seemed that Master Chen refused to teach Lee the Leaf Dragon God.

Later on in the series, it appears that Lee was able to use the move anyway. How is this possible? Did he learn it from somewhere else? And is this move part of Lee's cannon? Or is just me getting filler mixed up with cannon?
Its a filler episode. 

From here you can see that it is present in the anime only, with no reference in the manga. Also can you specify in which episode of naruto/boruto Lee used that move. Apart from the filler there is no mention of that move in any other episodes.

Also check here to see which episodes are filler and which are not.