Back in the day, before being spun off from Hitachi, Renesas made the CPU for the original Lego Mindstorms: https://en.wikipedia.org/wiki/H8_Family

They're a big deal in industrial/automotive/defense but they don't have a ton of visibility. Perhaps why they'd be interested in more outreach, Atmel and others like STMicro definitely reaped benefits from engaging with hobby/educational users at probably negligible cost. Arduino has also been trying to position themselves as a legitimate friendly platform for industrial automation, so it could be a good match.

Renesas is (or was, at least) pretty big in the automotive sector. They are much less common in general consumer electronics, though.

i have used renesas microcontrollers in probably 20 defense contracts fwiw

How open is the toolchain/SDK for Renesas parts? I.e. can they be used with gcc/gdb/(c)make with reasonably low effort (like esp32, stm32, rp2040) or is the only realistic option using their IDE?

it's all open source, you can see part of it here https://github.com/arduino/ArduinoCore-renesas

you can use GCC with https://github.com/renesas/fsp if you want to go very low level.

their official fat sdk is a run of the mill eclipse reskin w the usual gcc ports and half working libc options last i used it :-)

What defense area? I've done a ton of random aerospace stuff and usually end up using STM or NXP. All the critical systems have been some horrifying PPC or Sparc derivative.