Nowhere does acknowledging and permitting the use require a C&D. A simple “we see you and this is fine” letter would’ve also met the policing requirement.

The C&D is only required when you want to shut folks up and get the lawyers their bonuses.

Those websites were not selling chips or chip designs. Those websites used the trademark to refer the same product and do not create any potential for confusion.

Please show me where there is a "legal duty" to go after fair use and be callous and lazy towards someone who is actively working to make your product better.

Edit: Even if ARM had been completely unwilling to allow the fair use here, they could still have reached out directly since there is already a close relationship, rather than to going to the hosting provider and getting everything, even unrelated stuff, yanked down.

I sure hope this is the end of the Pro Bono work the author does for them.

This isn't one of those cases. This isn't a Xerox, Hoover, or even Google situation. Simple reference of a trademark is allowed. What holders need to guard against is dilution and generification within the applicable domain.