Check the timeline. It wasn't separatism that drove companies out, it was Bill 101. (OK, Bill 101 was driven by the same motives as separatism.)
Thanks to that bill, you had a population of professionals who had moved to Montreal for good jobs who found that, because they were born in a different province, their children could not be educated in English. These professionals found it less painful to move to Toronto and find new jobs than they did to watch their children be forced into schools that didn't speak their language. Companies found that it was cheaper to relocate headquarters than to lose these employees, and this accelerated the brain drain.
Quebec continues to be hostile to anglophone immigration, but most of the damage is now done and permanent.