Ex-IBMer here: until 2015, Macs weren't an option for people upgrading their PCs. They're deploying new Apples as laptops age out, and employees will then have a choice of a MacBook/MacBook Pro (depending on job role), a Lenovo laptop (configured for job role), or for the special snowflakes with sufficient budget, some custom configuration. BYOD is alive, well, and encouraged at IBM.

Most IBMers with Macs were using their personally owned hardware (as was the rare IBMer using a Microsoft Surface). Early adopters (before the end of 2014) were supported internally through the community, and starting in 2015 we were required to install a configuration program that would add an IBM app store and configure the machine so it complied with the internal security rules. Many of the day-to-day headaches on the Windows machines (e.g., printing) were eliminated with the IBM development Mac tools, but other things were impossible or required a VM to do, like using an IBM internal SOCKS proxy application to connect to customer sites.

IBM has a very active and dedicated Linux community as well. I can't speak to support numbers, but the tooling was more mature and more tools were available on the RHEL distribution.