I am not aware that Intel gave any credit that is visible to the end-users of their processors (otherwise I would surely have seen it, since my laptop has a Skylake processor - so it uses MINIX 3). I am also not aware that my laptop included a leaflet that mentioned MINIX 3 somewhere.

So my assumption rather is that Intel's lawyers reached an agreement with the license holders of MINIX 3 that they don't have to give credit (which is perfectly fine - even the quote in my parent post mentions this possibility). But if they did such an agreement they said that it is perfectly fine to them (in particular for Andrew S. Tanenbaum) that Intel does not credit them publicly. I thus consider it as rather unfair to complain when such an agreement exists, because one typically does not demand such an agreement if one plans to credit them nevertheless.

So I rather believe what happened is this: Intel came to the MINIX 3 developers to get them to sign such an agreement, which allows Intel not to give attribution. Andrew S. Tanenbaum signed it, because it sounded too good to be true and gave MINIX 3 some very renowned key customer. But he did not consider that this might put MINIX 3 into very common use without any users being aware of it. So he "suddenly" realized the loophole of this agreement and now considers himself treated unfairly.