Interesting find, I took a cursory look at their GitHub[1] and they seem to accept PR from outside but I didn't find any explicit mention of copyright transfer; Perhaps because there's no separate version of sequence.js for commercial use(Just use case differentiation).
> there's no separate version of sequence.js for commercial use
A separate version is "open core", not "dual license" - that's a different business model.
Use case differentiation by means of providing the same thing under different licenses is what dual license (or multi license) means. One license (say, GPL) will be appropriate to some use cases (downstream open source projects) but not for other use cases (downstream proprietary software) and so the latter has to pay for a license other than the GPL.
> I didn't find any explicit mention of copyright transfer
It's possible they know the people who've submitted code outside of github and handled it out of band, or that github has some way I've missed of requiring submissions to assign copyright in a way that wouldn't be visible to us. It's also possible they're being legally reckless and might be subjecting their paying customers to potential (if perhaps unlikely) copyright claims from their submitters. There is nothing about it being the same code under both licenses that would imply they wouldn't need their submitters' blessing in some form to sell their code under the proprietary license, and the more implicit blessing the more likely it'll wind up in court at some point.
> the need for having a permissible license for the dual-license.
A more permissive license generally improves adoption. For the same level of adoption, a less permissive license drives more revenue in the dual-license model. Maybe a permissive license is still the way to go (certainly getting adoption is often difficult), but it is not the dual license situation motivating that. Contrast a "give away the software and sell support" business model, where (sufficient) adoption is everything and more restrictions on the license doesn't do anything for the business.
With a maximally permissive license, I hesitate to call it "dual licensing", as it stops being the license your paying customers are actually paying for but rather hosting, support, or warm fuzzies.
> a less permissive license drives more revenue in the dual-license model.
But we've so far encountered just one example for that, Even there we don't know the financial status to claim whether it's indeed more successful (revenue wise when compared to a permissible license).
On the other hand we have numerous successful products (by revenue) with permissible licenses incl. those sighted in the OP.
