Renesas had been trying to get into Arduino market, previously with Gadget Renesas efforts. Arduino is expanding more into (over-bloated...) industrial control applications for heavy industries. There, I would assume, the both parties saw aligned interests to have a Renesas chip in the center of a new Arduino(maybe ARM might be in it too).

Performance must have been a non-issue, they make everything from timers for rice cookers to custom ISA smartphone SoCs. It must have been just an option that is good enough, easily available and comfortably low-tech to let to an enthusiast with an SEM in his basement.

Okay, now that finally makes some sense!

Though does Arduino really amount to something measurable to a giant like Renesas? And I presume that Arduino plans to keep the IDE and API, so not like 99% of people would be learning anything about the details of Renesas' chips.

Renesas/then-Hitachi H8 and Microchip PIC were popular in Japanese EE colleges and Universities. The H8 was always a bit too advanced, more popular in more niche robotics than plain EE, and must have been not too important for Renesas. But the latter was blown away entirely by AVR as Arduino came around and created a whole hobbyist industry.

IDE and API won't be issues, it's just bare C/C++ and APIs are just extra standard libraries. But the popularity of Arduino and ATmega328P/ATmega32U, its expanded talent pools, etc must be somewhat tempting and potentially-vital-looking for Renesas, while also not too tempting nor threatening to make substantial changes to its operations and focus on over the counter chip sales.