Why does impedance decrease in an MLCC as frequency increases?
MLCC capacitors generally have a much lower effective series resistance and therefore will have a sharper series resonance compared to Electrolytics/Tantalums: -

Picture from this wiki page.

&quot;this equation is not ( in math terms ) &quot;rigorously correct&quot;, it is
actually a crude &quot;rule of thumb&quot;, which has some built-in assumptions
that are NEVER identified, anywhere.&quot; on one of the answers here
Is this true?

I can't address your 2nd paragraph because it does not read correctly i.e. it is gobbledygook. Try fixing your question.
If you are referring to the answer given by someone called &quot;@Bob S&quot; to this question then take note that Bob S has not received upvotes for his answer, has accumulated a total reputation of 1 and hasn't been seen since April this year when he joined stack exchange. In short, his answer is not to be relied upon.