Woman in underwear in own home

It's the usual tactic to make an OP feel like they're alone / in the minority.

That said, I find the mums/wives/girlfriends/sisters/daughters comparison a slightly odd way of looking at it.

Why is that odd? Most people see the world around them through a lens of their own life experience, you only have to see the biased answers on here to certain subjects to know that it's a thing.

But for instance.. a lot of women I know would be mortified if they were chilling in their own home in their pants on the assumption they were in a private space, and a workman suddenly arrived who could see them, they would absolutely like to know in advance if it was going to happen again so they could make a decision on whether to take extra precautions to protect their own privacy when the workman was around.

Based on that I would assume you may feel the same, and if I knew it was something that could happen, I would warn you just in case you did feel that way.

Or maybe the neighbour got the impression that the contractor enjoyed getting a sneak peek.. again, I wouldn't make a thing of it, but I would absolutely tell you he had seen you to make sure you could protect your privacy if you wanted to.

The only time I wouldn't say anything is if I just didn't give a single shit who saw what and when and if anyone may be upset by it.. so IMO he is being neighbourly.

Obviously if you are genuinely not fussed about privacy that's entirely up to you, but I really can't see how has done anything wrong by just passing on the information he has been made aware of, via your husband (seeing as you weren't in).

Another alternative is that the contractor said something about it that the neighbour was uncomfortable with, in a more creepy or degrading way and the neighbour felt compelled to tell your husband to make sure you were aware that someone can see and give you the heads up. In fact this does sound more plausible now I’ve thought about it. But you’ll probably never know for sure.

You are having a very strange reaction to this. You believe that you are entitled to do whatever you want in your house, which means the comment should be of absolutely no relevance.
I walk around in my underwear and have no curtains. I assume some neighbours and contractors can see that, I give absolutely no thought to it. If a neigbour mentioned it to me, I'd just thank them and shrug it off.
But most people would want to alert you because they assume that everyone is terribly precious about others looking into their properties. Which is a reasonable assumption to make because many people are.
It's a complete non-issue. Nobody judged you, nobody suggested you cover up.