There was an awful lot of nonsense put about during the Operating System Wars, a lot of it by people who had not the first bloody clue about operating systems at all.

Sometimes it was a very clueless manifestation of the telephone game effect, where the fact that OS/2 API was designed to be easily callable from high-languages, without all of the fiddling about with inline assembly language, compiler intrinsics, or C library calls that one did to call the DOS API, could morph into a patently ridiculous claim that one could not write OS/2 applications in assembly language.

Sometimes, though, it was (as we all later found out) deliberate distortion by marketing people.

Patently ridiculous? Yes, to anyone who actually programmed. The book that everyone who wanted to learn how to program OS/2 would have bought in the early years was Ed Iaccobucci's OS/2 Programmers Guide, the one that starts off with the famous "most important operating system, and possibly program, of all time" quotation by Bill Gates. Not only are examples dotted throughout the book in (macro) assembly language, there are some 170 pages of assembly language program listings in appendix E.