If you're a gypsy brewer, isn't every beer a collaboration brew?
It's a collaboration brew when two or more brewers (**drumroll**) collaborate on it.
When a gypsy brewer rents someone else's brewery to brew a beer, it's just a brew.
In the case of lambic-based beers, things become a bit muddled. Anyone can purchase young (and presumably old as well) lambic with some if not all of the lambic brewers. Girardin for example will readily sell 30 liter bags of young lambic to consumers and homebrewers alike, who are all freee to use it as they please. Courtesy demands that you mention the lambic brewer in your final product (often as a badge of honor, as in "...with Girardin lambic, not just any old homebrewed sour, you know?") but I don't think it's even mandatory for commercial brewers to do so. Not to even call the final product a collaboration brew, because, looking at it sideways, the lambic brewer played no actual part in the conceptualisation of the final beer, besides provide a base ingredient for it. I know, it's sophistry, but there you have it.
I recall a beer by Dillewyns which was a blend of their Vicaris and Girardin lambic, which they called Vicardin, which was about as formal as the nod ever got. What I'm saying is that labic blends are a bit of a grey area in terms of collaboration. In the case of the Spontanbasil, I'm guessing Mikkel and Lindemans came to an agreement to launch it as a collaboration brew.
That being said: many of Mikkel's beers are brewed at the Proefbrouwerij, who happen to have both the skill and the capacity to brew seriously wicked lambics and sours. The fact that this one comes from Lindemans instead is perhaps telling, but I can't imagine what it's actually trying to tell.