I think you are too serious about the meaning of that word.

I think there are some explanations of seriousness dropped here and there in the article, and it is not exactly what you mean. it is rather used for following rules without questioning, respecting discipline to the detriment of creativity.

this means someone can be serious in their work (in your terms) without following the seriousness of the subject (in the author's term).
he suggests playfulness for example to break well known dichotomies, or an other example to adopt foreign ideas instead of translating everything to familiar terms.

at least this is my recollection.

>but being playful can be limiting in itself, especially in a complex topic, like engineering or empathizing with trauma, where care is absolutely needed in order to make much progress

but he is talking about philosophy, which is inherently general, not as specific as a given engineering problem.

btw the most exciting products of engineering are always the ones which are attempting to break paradigms.

in Hungarian "fika" is the rude version for booger