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Yes they are, facts are not arguments. Facts are facts and prove or disprove arguments.
A fact is that most operating systems in use today are either of the unix lineage or related to it, so the fact that C is being used, is a historically contingent point. If I choose to write in Common Lisp, my programms are portable enough to run on entierly different operating systems, and entierly different compilers/interpreters. Hence, Lisp is not "dependent" on C, because there is no causual necessity between that statment and the observations that C is commonly used to implement or bootstrap compilers, on operating systems inclinded to C to begin with.