Redis Ltd. broke the social contract. They decided that the short-term profitability of the company was more important than the project as a whole, the community which had grown around Redis and the future of the software. Probably a wise decision from a business perspective, but that doesn't make it any less of a rug pull from a community perspective.
You're right that no pre-relicense code was lost. People could and did fork. But something far more valuable was lost: the community's trust. First they relicensed the code solely for their own benefit, now they tried to take over third-party libraries and started behaving in a hostile way towards the community forks. Why would anyone volunteer their time and effort when it is mainly going to benefit a company which is so openly antagonistic against its volunteers? After all, what's the next shady move going to be?