It's physically impossible to swim through molasses. The analogy is either a failure or an insult.
Is it? Syrup is swimmable: https://mythresults.com/swimming-in-syrup
Molasses is a thicker concentrate, and is infamously deadly when it floods.
Everything is deadly when it floods, with water floods being responsible for about half of all deaths from global natural disasters [0].
The Wikipedia article you linked to describes the event but says nothing about swimming through it. There's a Scientific American article that analyzed this based on the Reynolds number [1] and arrived at a conclusion that you can't swim through molasses via regular symmetric motions and would need something different, which sounds quite appropriate for the analogy.
[0] https://www.bmj.com/content/383/bmj.p2101
[1] https://www.scientificamerican.com/article/molasses-flood-ph...
reading this : reading books intended for transmitting information = swimming through molasses : swimming through water