Monolith Soft had 70 employees in 2011. They now have 243

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If I had to guess I'd say one of their three secondary studios is handling XBC:DE while the main studio makes that new game they teased in 2017, and the other two help out with BOTW2 and other EPD projects.

It's not impossible XBC: DE is being handled by an external studio, but given Monolith have the capacity to do it themselves while still working on a new game, internal development seems more likely.