Six Sigma was just the means to an end. It helped manufacturing and supply chain get more visibility and standard tools/processes/mgt support to improve. Like anything centralized, Welch/GE could not apply it to some and not to others - requires too much work to understand the details of each job.
So what if some had to do green belt projects that didn't make sense (e.g., secretaries complained about this). But they learnt some basics, tools and skills to analyze problems.
Immelt's point was that he would rather take bold bets on technology, rather than processes in existing businesses. He was a visionary, surrounded by bigger visionaries like Ruh and Comstock who promised Google-like future for GE as tech company. Strategy without execution is just a dream, which turned into nightmare when Fossil fuel power industry slumped. If GE Power did not get caught with pants down, Immelt would have pushed the narrative few more years, declared success and retired. Many tragic ends to companies involve over-reaching into new areas and neglecting paying the bills.