Earlier this year Bloomberg double-downed on it's infamous 2018 "The Big Hack" story with "The Long Hack", https://www.bloomberg.com/features/2021-supermicro (Oddly it seems to have snuck under everybody's radar, including on HN.)
In The Long Hack they not only reiterate the same claim regarding SuperMicro motherboards, they reference a 2010 criminal case involving counterfeit Cisco hardware in which a witness (Marine Staff Sergeant on active duty at the time, thus speaking for the military) discussed supposedly chipped IBM-branded Lenovo-built laptops sent to the military circa 2008. (The counterfeit Cisco hardware case itself seems a less interesting example as it simply might have involved trojan'd firmware, which is sadly quite common.)
I've often thought that if they were going to do it, we would have heard about it by now.