I decided to go watch the video and didn't feel spoiled at all. I agree with VampireNaomi on this one.
It seems to me that the footage confirms that Lupin and Co will not directly interact with Goemon, only
watching him from a hideout. This would further confirm the spin-off movies do try to stay coherent
with the first series, since Lupin nor Jigen officially met Goemon before series one, in the anime "canon".
But that new movie would slightly alter our vision. It states that Lupin and Jigen already saw Goemon and
observed what he was capable about. And that would be their first reason to actually approach Goemon in the
green jacket series (Fujiko already knew Goemon from Mine Fujiko To Iu Onna).
I still think they are a few incoherences, but it is not the main interest from those spin-off movies.
And indeed, it really looks like Goemon is the actual protagonist, not in name only. ^^