The prevalence of the 21 cm wavelength is something they would likely discover should they be in a part of the universe similar to ours. What I find laughable is the means of trying to communicate this wavelength and using it as a means to indicate our size.

Now if you go the other way, referenced 21 cm as a well known quantity (say, by making the plaque 21cm wide and referencing its diameter) and used that to describe the hydrogen spin flip in order to teach or communicate our level of physics understanding rather than depend on a shared understanding, then I'd say it makes more sense.