In another two interviews (pasted here), he confirms that the Bumblebee Movie is in the existing continuity because he originally wanted to include Megatron in the Fall of Cybertron scenes, but conceded that he couldn't be there because he was currently frozen on Earth.


“I had this whole thing boarded where we’re we see [Decepticon leader] Megatron and he comes in like Sauron, just blowing sh-t up and laying waste to everything,” It was at this point in the idea that he realized Megatron was frozen on Earth during the time period which Bumblebee takes place (the late 1980’s) as revealed in 2007’s original Transformersmovie.
So even though I really wanted to see G1 Megatron on the battlefield at the fall Cybertron, I couldn’t do it,” Knight said. “But I wanted to fit as much of this G1 stuff I loved in there, and so I put my favorite characters in. Obviously, there are ones I wish I could put in there, and I would love to see an entire movie about the fall of Cybertron because I think it’s awesome, but just to be able to visit it for a moment, to see the Cybertron of my imaginations on the big screen, it was a real thrill.”

(it also says that those Cybertron scenes occurred in "present day", and not millions of years ago when the Cube was launched into space after Megatron first conquered Cybertron... so should the Movie universe characters Ironhide, Ratchet and Arcee look Gen1-ish and have Gen1 colours, since those scenes were 20 years ago, and we've seen them in pre-history scenes or historical pictures in their Bay Universe forms?)


This also means, that if Knight has his wish to do a Fall of Cybertron movie, the main bad guy of the franchise (Megatron) won't be in it... except if they rope in some flashbacks just to include him (and his toys).