Not super surprising. IBM's recent acquisition trends have been around trying to establish a beachhead in the cloud world. Their attempt, a few years ago, at buying Softlayer didn't work out super well (they still lag way behind Amazon, Azure, Alphabet and Alibaba). They've probably looked at Oracle and said, "being a meaningful CSP is probably a gone opportunity," and decided, "better to get a beachhead into all of them." Fits better with their consulting-heavy model, any way.
IBM's been supporting Linux on their systems since the early 2000s. You could get "pure" Linux on LPARs or, if you preferred to stay on AIX and just dabble in Linux, you could run AIX with Linux extensions starting with the AIX 5L release. Plus, if the Power-series wasn't your thing - and you wanted to stay more on the "big iron" type of hardware - IBM's been doing zLinux for quite some time. Whether PowerPC or z-series, IBM always verified Red Hat support on their Linux-oriented iron. So, not a completely out of the blue nowhere move.
And, given the alternatives, I'd really rather a consulting and semi Linux-oriented company like IBM buy Red Hat than company like Oracle. Then again, if Oracle was sniffing around, I gotta think that the Red Hat execs informed them "if you buy us, you can damned near count on every last one of the technical people heading for the door as soon as their ESPPs are able to be cashed out."
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