Quote from OP's link.
Mrs Foster's barrister David Ringland QC said a solicitor for the DUP leader had contacted Dr Jessen on Twitter instructing him to remove the post on 24 December but he had refused to do so.
Mr Ringland said the tweet was not removed until 7 January, when Dr Jessen responded to follow-up requests in a "pithy email".
As you can see, Jessen saw it and responded to it on 25th December.
If you're using a screen-reader, the conversation goes like this, below Jessen's defamatory tweet.
Paul Tweed, Lawyer for AF: I am putting Dr Christian Jessen on notice in relation to a totally false allegation he has tweeted regarding DUP Leader and former NI First Minister, Arlene Foster. Legal action will also be taken against any persons who have retweeted this highly defamatory allegation.
Some smartarse fan of Jessen's: Hi Paul, your reply has also pushed the original tweet through to your 12.3k followers. Everyone that liked and RT’d you has also pushed original tweet to their followers.
So far you have generated over 100,000 impressions of @DoctorChristian tweet.
Who sues you? @DUPleader
Dr Christian Jessen: "lol".
I hope Jessen thought that lol was worth £125k