My personal take on it is that you had reason to believe it wasn't confidential, as you were told they were aware of the plan. Equally, you were asked to review something which was inaccurate, and so it was important it was corrected.
However! Whilst I don't agree that 'of course' strategic plans are confidential (every organisation I have ever worked with has published them, and strategic plans are usually well known to competitors as well), I do think there needs to be an element of common sense in which elements of it you share, and when. A department being made redundant *could* be sensitive if it has implications for partner organisations, workload, governance, relationships, brand perception etc. Therefore it's the sort of thing that - whilst the plan may not be completely confidential - I'd have checked it was okay to share, before sharing outside the organisation. Equally, financial information would be in that category - I wouldn't share it outside the organisation until I was certain that was okay to do it, as that could have significant ramifications for a business.
However, I don't think they could argue breach of confidentiality - you were told it had been shared, it turns out there were two plans, and you were asked to check a document which was incorrect so you corrected it. I think they could argue breach of common sense, though.

Personally, I'd just be apologetic, explain the situation, and leave it there. You'll be leaving anyway!
' <-- See that? It's called an apostrophe. It does not mean "hey, look out, here comes an S".