Sun's business model was selling hardware: beefy workstations that you do Serious Computing™ with, unlike those wimpy home computers like PCs.[1] Later on, when x86 actually became decent and competitive they shifted to server hardware, but still: monetary units for hardware, not software.

And oh yeah, with this hardware you also got this SunOS Unix thingy. SunOS was very much core to Sun's success especially in the 80s (not the least because Bill Joy was already a "big name" even before Sun on account of creating BSD and vi), but it wasn't their core business. From their perspective they weren't really "giving things away for free" because they weren't selling it. Interoperability also gave Sun advantages as their SunOS/Solaris machines could communicate with other systems so it made the barrier to adoption/migration lower. People talk about this WSL thing on Windows now, but for Windows NT and 2000 there was SFU – "Services for Unix" – which pretty much did the same thing for Microsoft: make it easier to use Windows with other systems.

Microsoft in the 90s on the other hand was very much in the business of selling software. I'm not sure if they were making any hardware in the 90s (maybe some mice and keyboards?), but if they did it was a little side-thing and not their core business.

Sun was never really an "open source company" in the way we would understand that now: parts were open, many parts were not. They did eventually release OpenSolaris but that was much later. This wasn't entirely Sun's choice: the legal situation at the time was complex, and even OpenSolaris wasn't 100% open due to legal reasons (see the BSDi lawsuit, or the SCO lawsuit that is hopefully finally dead once and for all).

It's similar to Apple today; you can still download the source for Darwin, Swift, Safari, clang, and some other things. What does Apple have to gain by keeping these closed? It's not their core business: no one "pays" for Safari or even macOS; they pay for iPhones and AirBooks. There is still a lot of "secret sauce" at Apple – as there was at Sun – and you could argue if that really protects Apple's profits or not – but there's a wide agreement that "some openness" doesn't really hurt their bottom line.

It's not a coincidence that Microsoft has become "more open" as their business model moved away from almost exclusively selling Windows, Office, and other software suites to selling Azure Cloud, Office365 subscriptions, X-Box, and other things.

In short: Sun never really "gave away" things, they just had a very different business model than Microsoft. On one hand you could say that the author of the email doesn't really seem to appreciate that, but you could also say that Sun failed to foresee that their core products and business model would slowly become obsolete. This is easy to argue with the benefit of hindsight, but less obvious in 1996: they had $7.1 billion revenue/$476 million net income in 1996 (Microsoft had $8.6 billion revenue and $2.2 billion income in the same year; less overhead if you distribute only software) and in 1997 this increased to $8.6 billion/$762 million. The profits continued to increase until the dot-com crash, which they recovered from only to be done in by the financial crisis.

[1]: https://dilbert.com/strip/1995-06-24