Different resources say different things, and no, that link you sent does not say "A stands for accent" as simply as Zach is smugly saying it does. ;)
In fact, the text he is referencing is discussing how you might use SECONDARY color palette colors to accent those things. Other resources outside of Google say that the A stands for "accent" but the Angular-Material documentation says that a combination of primary values without the A as well as one or two with the A are best used for non-accent while some secondary values are best for accenting including some A#s. Basically... it's not clear. Set those values as you please. It doesn't seem that, without diving into the code, you will ever fully define what the "A"s stand for.
this is a screenshot of the full text that Zach grabbed his answer from... clearly referencing that secondary colors (not A-colors specifically) are used for those types of components to create an "accent".
In the end... "accent" and "accent colors" and the "A" just don't necessarily line up, and are not clearly defined anywhere.