Traditionally, one of the best paths for an upstart company to challenge an established giant was to develop superior technology. Indeed, this is how some of the cloud giants themselves got their start against the dinosaurs of the day.
But in a tech culture where open source is not just common but expected, this path is severely hampered. A lot of tech users won't even consider your fancy new technology if it isn't open source, but if it is, then the giants can put it on their platform and make your customers into theirs.
This is what "Microsoft loves open source" means. When they had the (commercially) superior tech, they disparaged open source as nonsense so they wouldn't be asked to share. Twenty years later, when they realized that there was more and better tech out there than they could hope to produce, they aligned and supported that very ideology so that "we have Redis" or "we have Akka" wouldn't be a competitive advantage anymore.
Ultimately, it's a twist on the old "commodify your competition".