Actually there is a large historical overlap between the two communities -- warm, collaborative, always giving credit where due. Different goals, lots of feedback both ways.

Jens Mönig was on the Scratch Team (invited by Mitch Resnick). BYOB was presented at Scratch@MIT 2010 explicitly to merge ideas back into Scratch, not to fork the community:

https://scratched.gse.harvard.edu/resources/announcing-byob2...

Berkeley has kept showing up at Scratch conferences (Amsterdam 2015, Bordeaux 2017), and the Scratch forums hosted BYOB/Snap! discussion for years:

https://scratch.mit.edu/discuss/topic/4455/

So "NIH syndrome," "reinventing the wheel," and "fragmenting the community" is pretty much the opposite of how these two actually interact.

Documented cross-pollination:

BYOB => Scratch: custom blocks (Scratch 2.0 took command blocks only, not reporters/lambda). That was an explicit goal:

https://en.scratch-wiki.info/wiki/Snap!

Scratch => Snap!: browser rewrite timing influenced by Scratch 2.0 plans; Morphic via John Maloney; CC-licensed costumes/sounds used under license.

Shared people: Jens (Scratch Team => Snap! lead), John Maloney (Scratch/Morphic; GP session with Jens at Scratch2015AMS), Bernat Romagosa (Snap!, MicroBlocks, Snap4Arduino -- Bordeaux, Snap!Cons).