Right now, its not possible to quote a post from a locked topic because we cant reply. Quoting from locked topics could be useful sometimes to refer to a normal quote like its possible for every unlocked topic. I know, its also a solution to just quote the text and add a link manually but that wouldnt include the date of the quoted post and need more time to type the author, especially for multiple quotes.
I am in favor of enabling quotes in locked topics. But you can use
epochconverter for adding the time of quoted post.
Enter the exact time the post was created and click on "Human date to Timestamp" button. Then Epoch timestamp will be shown.
Here is the code for quoting the post you referred to.
[quote author=theymos link=topic=5193860.msg53152306#msg53152306 date=1574458668]