In the US, it would be completely, totally normal and legally acceptable that a company might have a policy:
"You may not smoke while on video calls."
However. It sounds like they have never told you this policy and indeed they probably do not have such a policy. Your email response should be:
"Hello. Immediately completely remove the reprimand from my record. Reply with written confirmation that this has been done. Secondly, please provide me with full documentation showing the company policy which prohibits smoking on video calls. In fact, I believe such policy does not exist? Thirdly, if it does exist, please provide me with full documentation on when this policy was enacted, by who, and most specifically how I was notified about it. Again immediately remove the reprimand from my record - and confirm that this has been done - until points 2 and 3 are addressed.
(Of course, again, if they do have such a policy on file - and it was communicated to you specifically or generally - they're in the right.)