You wrote:"Non-compete agreements are rarely held to be valid as they restrict employment."
This is patently false.
The correct answer is: "It depends.".
Non compete contracts do hold water if written properly. If they are overly broad, and they restrict you from working in your field of expertise, then yes, they are unenforceable. However, if they are written correctly and limit your ability to go to work for a competitor for a reasonable period of time, then they are enforceable.
An example... Doctors routinely sign non-compete agreements that prohibit them from starting a competing group within an XX mile radius of their current group, within a year's time. Trust me, this type of contract has been enforced.
Without seeing the contract, its impossible to know how enforceable the contract is.
If IBM specifically listed Dell and/or other competitors, then it could have been enforceable were IBM to have correctly plead their case. They lost because they didn't outline what he could have known that would have been interpreted as 'trade secrets'.
The litmus test would be to see if the contract would have allowed him to work for any tech company in an M&A position. If it named Dell, Apple, Oracle, and Microsoft but didn't name Cisco, Seagate, SAP, Intel, HP, etc ... then it would probably have been enforceable. Also it would only have applied if his job title/responsibility had been the same. Had he joined the company as their CFO, then again probably it would not have been.
With respect to the new allegation, its an interesting one. IBM frowns on its employees starting a side business in a similar field. So a techie couldn't do a side job as a consultant. He could however start a business as an owner of apartments for rent. (I know of a couple of IBMers who have done this.)
All in all, I do think IBM is actually scared about his departure.