Thanks Rob for some comments.
I used to use espruino at the very beginning - I used to test the STM32VLDiscovery board many years ago and used to report issues directly to Gordon for a few months.
At that time I used to develop nokia-phone based GSM alert system (home automation and boiler temperature control). And the event based-design was pretty cool to handle FBUS communication.
Now I see that the product is much more mature and decided to play with it again (after a few years). I have not decided what particular espruino hw suits me the most yet.
I do have the mentioned keyboard and it works fine with the smartphone and Windows.
The chipset is very common - Broadcom BCM2042. I would say it's typical design of BT HID keyboard.