I would liken this to saying operating systems based on Linux are open source, except when you start adding binary blobs like nvidia or printer drivers.
You can recompile Espruino to not use bluetooth (for example the stm32 ones don't have bluetooth) but the watch won't be as functional (e.g. no app store, no wireless comms).
Nordic is the manufacturer of the microprocessor that powers the watch, and they also provide the bluetooth stack. Since they are the manufacturer, the Nordic provided bluetooth stack works best with the chip, similar to how nvidia drivers have better support than the nouveau reverse engineered drivers.
Another tricky thing for opensource products working with wireless comms would be compliance. If you use the Nordic sdk, you can mention that in your product docs and the certification officials would know what to do (since a lot of products with the Nordic chips would usually use the sdk and they would have experience with it). If you don't, you're in for a more grueling (and possibly more expensive) certification process since you'll need to ensure your wireless energy signals don't go beyond what local laws require. Since Espruino can be used in consumer products (the only one that comes to mind right now is https://lab.ruuvi.com/espruino/ ) the product manufacturers benefit from this as well.