Suddenly the problem got a solution.
When the tablet was powered off (USB cable should be unplugged for that, otherwise TWRP powers the tablet always on), I held Volume Up and Volume Down and pressed the power button.
In this case not the TWRP, but the standard Huawei downloader comes up, with an error message that system upgrade could not be performed (of course! I didn't put anything to the SD card). It had the only reboot button.
After tapping this button the system boots normally.
Because all the data was wiped out, it needed some time to initialize, but then worked normally.
I had to configure Wi-Fi, and the system found an update from my current version ... to the same version, with the wording that it was a system restore:
Restore system to latest official version.
After downloading the system asked for a restart, rebooted and landed again in TWRP.
I repeated the trick with three buttons, let the system to download the update, held Volume Up and Volume Down physical buttons and only then tapped on the "Instll now" screen button.
It caused the stock recovery screen to appear. I tapped the reboot button there, and the update rolled out.
After the update was complete, the system boots normally (except for the unlocked bootloader warning) and if volume+ button is held while powering on, the default recovery dialog appears. No traces of TWRP remained.
