I'm struggling to see what fallacy there
The "OS/2 fallacy" is based on the idea the product failed because of Windows compatibility - mainly that developers were disinclined to develop native OS/2 programs and that tanked the OS. This is provably false (it was a popular feature), and the success of Steam is the rebuttal to this. OS/2 failed for a variety of reasons unrelated to this, mainly poor management, poor marketing and an "IBM culture" that wasn't conducive to developing a consumer-friendly product.