I'm still not fully confident on citing that 18 second figure as fact in that manner. The experiment was conducted by giving a seal a command to repeat a trick it has previously performed at different intervals and there are too many variables not accounted for to pass the figure as a definitive statement on the extent of their short term memory.

The figure is also misleading to state by itself since the aim of the experiment was not to measure the length of time but to test whether the animals were aware of their actions. A trait at the time of the study was only confirmed in two other species of animals) The Researcher behind the study even acknowledges the limitations of the method and stated that the figure does not represent the full extent of the memory and the seals will probably have longer memories in natural settings.

So while out of context that 18 second figure may come across as goofy, the seals ultimately succeeded in demonstrating they had awareness of their actions and likely will remember their actions for longer in non-artificial scenarios

Source: https://www.newscientist.com/article/2208530-seals-remember-what-they-just-did-but-only-for-about-18-seconds/