from my point of view, you have to ask this question on the Arduino forum or Arduino developers.
New target is usually ported (to Mbed) by its owners/creators.
- Portenta H7 is based on STM32H7 series. So Portenta inherits from the source what was already done by the ST here under the Mbed.
- Arduino Nano 33BLE is based on nRF52840. So that also inherits from the source what was already done by Nordic (or someone else, I am not sure).
Arduino DUE is based on an Atmel Cortex-M3 and there are no Atmel currently supported. There were supported some Atmels in the past, but not Cortex-M3.
Port process overview - Porting | Mbed OS 6 Documentation
What I read about connection between Arduino and Mbed, the benefits are only on side of Arduino. Arduino probably can use many/all Mbed boards under its environment, also with its libraries but not vice versa.
I’m not a professional but that’s how I understand it.