That doesn't mean anything. It was in development for nearly 2 years when it was announced at E3 2019. Everything is "under development" until they release. They just didn't commit to a year date because they're not sure if it'll make 2020. It's using everything from BotW and by this holiday it'll be 3.5 years of development. That's plenty of time to put a sequel to BotW out.
"Using everything from BotW" might not be entirely accurate, we don't know how much stuff will be new either, it doesn't seem unreasonable and Nintendo doesn't just release games because their development was complete.