I don't remember where I got it either but with essentially only three engine manufacturers (Cummins, Detroit, Caterpillar; Mack makes their own engines), why would 40% be so unrealistic? What do you suppose Cummins' market share is now?
Firstly the USA is not the entire world and the USA imports a lot of trucks , particularly in the 2 to 6 ton classes which are fitted with foreign made engines.
You also omitted IH who own the Navistar diesel plant that supplied engines to Ford , Dodge and a variety of agricultural factories.
Perkins which while owned by Cat still makes diesels in the USA for off road & Argentina for road use and in Japan for cars.
Ford had 3 diesel engines it made itself although down here they were not that popular.They also fitted Navistars & Cummings
The F series down here ( so I suppose in the USA as well ) was available with a Ford branded diesel engine although they were very similar looking to the Iseki diesel engine , possible a rebadge.
Europe used diesel engines almost exclusively for all trucks over 2 ton and a lot of these got imported into the USA.
SO Cat many have had 40% of a particular segment of the market but not entire diesel engine market.
And lets not overlook off road diesel engines used in rail, marine, mining, generators and of course road plant tractors.
When Cat took over Perkins in the 80's back when all CEO's were innoculated with the idiot "own the market" horizontal intergration idology Cat claimed to be the biggest diesel engine maker , IN THE WESTERN WORLD, convienently ignoring Russia, China Etc.
It would be a close call, but I rater feel that the EU would make more road diesel engines for trucks than the USA as a lot of US trucks still run petrol engines & Europe has been almost exclusively diesel since to 70's