Your question was: Is DH being unreasonable with his theory?
"DH thinks it surely is performance related."
"I actually don't think it's malice and more about lack of knowledge of the corporate world"
These comments contradict somewhat. If he lacks knowledge of your industry, how could he possibly be informed enough to suggest that you were made redundant due to your performance? Has he explained why he thinks that? That aside, if it were performance related you would be dismissed, not paid redundancy. Does he not have experience of workplace procedures?
"It can be read that wayabf I kind of do BUT he also says I don't need any help with CVs, CL etc, and that I'm extremely capable of finding another job and quickly"
That reads like this is your problem, get on with it. It is a bit dismissive. Did he say that then put his headphones on?
"From a moral POV I wouldn't do it though. I'm happy to leave, and my colleague is about to get married."
From a moral POV, all is fair in love and war, and you and your career matters just as much as the other guy. Why are you happy to leave?
I think your DH has no place to be having a theory, and even if he did, he should keep it behind his teeth.
A supportive response from DH would look like, "I'm so sorry, I know how hard you worked there, and how excited you were about your promotion. It sucks that they have had to sacrifice you to make the budget. How can I help you? Do you want to talk about it? Do you want me to get takeaway and wine? Would you like me to help with your CV next week, but no pressure for now, let's just chill and slag off your boss.
Unreasonable is stubborn, he's just being a dick. I hope you get an amazing next job that makes you super happy and fulfilled. People don't get promoted for no reason! You clearly are great at your job.