Because even the author references competency-based views of competitive advantage, but for some reason ignores resource based views, and ignores the fact that companies might be aware of their competences. That is to say, I'm sure that large companies tend to mostly be aware of what their competences are based on the resources and knowledge that they have. If they don't have marketing departments that have analyzed the ERP market, sales teams with ERP training, tech departments with key HR, key knowledge etc etc, then I'm certain they are very well aware of this.
Maybe some companies have had marketing missteps and have made poor strategic and competitive decisions, however, but I really doubt that it's due to a lack of introspection or simple analysis as described.
Also, IBM didn't "think nothing much" of the software layer. They misunderstood the nature of power in the supply chain, and most importantly, didn't solidify their position within the supply chain while they were dominant.