I think that without a transcript of the email sent to you, it would be difficult to see HR's angle on this. You said
>> Today, I got a formal notice from HR that I was receiving a citation for smoking in the office.

Can't edit my post as it was too late but...

And you said:
"They said that because everybody is working from home, my home counts as my office while I'm working."

You could argue that they don't pay the rent, utilities, tax etc...for your home so it's not their office (unless they do pay for any of those).

If you're smoking around equipment provided by them it'd be bad form, as I've seen what smoking around electronic equipment can do to the internals. So they'd have a point there.