Except for the self deprecating clause I think that's a very constructive rebuttal.

I question whether someone with claustrophobia really is likely to be "more than happy to help out" in this case. Speaking of one's problem as silly while in the very environment that may trigger an anxiety response sends a conflicting message, casts doubt on the sincerity of one's words and unnecessarily ups one's baseline anxiety. Genuine empathy towards another distressed flier doesn't seem a stretch of imagination here but rational self-interest will likely dictate one's response more than any other factor.

As someone prone to panic attacks during periods of prolonged confinement I empathize with part of the OP's predicament. Building on Stumblefoot's useful approach, another reply to the FA might be: "I'd like to help out as long as I can be accommodated in another aisle seat. What specific seat did you have in mind?" If FA doesn't know offhand or simply presses harder for a unguaranteed favor then calmly inform them "I'm claustrophobic and cannot tolerate middle or window seats. I'm sorry, but my answer is 'no'." It's polite but final and should be perfectly acceptable without further discussion. Return to whatever you were doing before the request or just sit back and close your eyes. (And repeat to yourself as necessary: " 'No' is a complete sentence." ) If the FA continues there's a bigger problem at play and one that probably has little to nothing to do with you. How and whether to escalate the FA's problem to the purser, the pilot or a gate agent is another thread for another time.