Opportunity cost is an open-ended economic term, not some obscure math or physics arcana, and it's core is all about preferring one option over another after an assessment (of what you lose by not going with the other, etc).
Using it casually is totally compatible with the concept, it's not like using hard math (e.g. Godel's incompleteness theorem") in some pseudo-scientific fashion.
>To your point, everybody lives life this way, there's really no other way to live life than to make judgements, so why respond w/ that unless you're actually making some more targeted point about only learning things that provide you the highest utility.
Everybody uses O/C in some way or another in their lives (it's not some obscure idea of only academic interest), but not everybody uses it well.
Many waste efforts on multiple things ("jack of all trades"), others chose the thing that makes them worse off professionally or personally over better options open to them, etc.
So, that's the point I was making: choosing math over Chinese (as the author suggests) can work out well from a O/C perspective. Understanding math better is more approachable than learning a language like Chinese adequately to make anything of it, and can have a wide impact (especially to a programmer, but also in personal life, business decisions, etc).