What you say is technically correct, but in the sense where you can put rat poison in One of those ceramic cookie jars they sell in houseware shops. There is nothing wrong in doing it, but it may lead to interesting failure modes Because someone can have implicit assumptions about what’s in there.
they are misleading if you assume unimodality, but are always relevant. If you care about how many modes there are then likely you would prefer deciles or centiles.
But even in the first image of the article the fact that two quartiles are close together means that there some density peak around there.
I agree with the author that box plots are not good plots, but quartiles/deciles/medians are useful even for multimodal distributions
No they don't. They show quartiles mostly, and don't assume symmetry or any parameters of a gaussian.