Re: Is there any reason to suppose this latest accusation is any more plausible?
Not really: There is no formal statement from Microsoft Corporate mentioned in the article or the posts, just an assertion from an individual - a former contractor to MS as of this year - though he does reference the opinions of more senior people that have worked on the kernel.
The post does mention that he thinks MS probably does not care given that "ReactOS aligns with a very old version of the NT kernel".