I'm extremely dubious about this claim and cannot think of any mainstream OO language this applies to (clearly not C++, Java or C#, nor of course Smalltalk, hence neither Objective-C nor Self nor Javascript).
For further context: I learned about programming languages, did programming language research starting in the early 90s. CLOS, and multiple-dispatch especially were always a significant outlier.
Simula, Object Pascal, C++, c#, ... (class based OOP with virtual functions)
Smalltalk (message passing, classes)
Lisp/CLOS (generic functions) -> Dylan, Perl, Julia, R, ...
Actors, Self, JavaScript, ... (Prototypes)
....
https://interlisp.org/saved.html#history
Besides Common Lisp, the only languages that have anything on their object model that resembles CLOS are Dylan, Clojure and now Julia.
Regarding R and Raku I cannot say anything, but the CPAN package for OOP extensions I used back in 2000's wasn't it.
But Moose is largely influenced by CLOS.
https://www.perl.org/about/whitepapers/perl-object-oriented....
some comment here on Hackernews :
"Moose is back-ported Perl6 objects, which borrows heavily from CLOS and the concepts introduced in the Smalltalk traits paper. This is a bit of a simplification of the history, but the ancestry is present, documented and obvious."
Lisp probably counts as the first major language to have OOP, and along with Smalltalk, was the language that introduced OOP to the wider programming world. Bjarne Stroustrup knowing about Simula (which was always a little esoteric) and trying to turn C into a Simula-like language, is more the exception. Most OOP languages today can trace their OOP heritage to CLOS.
[1] https://en.wikipedia.org/wiki/Common_Lisp_Object_System
[2] https://en.wikipedia.org/wiki/Flavors_(programming_language)