Can I get some context sorted out please, before I read that post carefully? Is the phrase 'Open Source' being used as if synonymous with 'Free Software'?
I'm fairly sure the difference is almost entirely their different business models; or maybe one has a business model and the other has a social model that very few people have ever taken any notice of. The difference is why 'Open Source' exists - a political split between the socialist and libertarian camps of Freedom. I never understood why it was given such a misleading name, as I understand commercial software that gives read-access to source code does not meet the definition, because 'open' really means 'free' (to do with as you wish with it.)
If RMS had been given (really) Open access to a printer driver's source code, he could have backward engineered the spec and replaced it by his own version that he cold hack on, none of this need ever have happened Yes, I'm ignoring software patents, in order to hold onto the last threads of sanity but Copyright vs Copyleftish co-operative software teams could have happened within the capitalist system; preventing over-charging, by allowing competitive development. We only actually needed published specs for all commercial software to prevent monopolies, with access to code when anything was unclear. Commercial software companies that only charged what the effort of producing their products was worth, would have been safe.
I think we must conclude that RMS wanted something else and he's a very clever strategic politician.