The article "Thermal Facts and Fairy Tales" [*] starts with a bang: "...a fixed temperature boundary condition in a thermal simulation represents an infinite heat sink." Fair enough. I always understood this, but have never seen it represented so succinctly. Having been educated as a chemical engineer, my thoughts immediately went back to my days hovered over a copy of BSL, aka Bird, Stewart and Lightfoot, aka the little red book "Transport Phenomena", which for decades was pretty much the only textbook on the subject used in undergraduate ChemE programs across the country. (Can anyone name a similar book in any subject that was so canonized?) A central tenet of the book was the similarity of heat, mass and momentum transfer.
So taking the opening statement and applying the basic concept in transport phenomenon to it, you can make the sentences "...a fixed concentration boundary condition...represents an infinite flux sink/source" and "...a fixed momentum boundary condition...represents an infinite stress sink/source", both of which I think are far more obvious than the original sentence.
[*] What am I doing reading "Electronics Cooling"? Well, I have done work for clients with heat transfer problems. The clients are mostly electrical engineering companies. I don't know if EE's are taught heat transfer at all (I suspect not) or they just don't pay attention in the class because they want to focus on circuits, antennas, IC's,... all the things that they went into electrical engineering to study - not heat transfer. You can infer the lack of understanding from the sentence quoted from the article. It only refers to a heat sink, even though the boundary could also be a heat source.