The Five Meanings Of #. And What Does ${####} Mean?

There is a mid-level way to "fix" this under zsh, as it supports nesting parameter expressions¹. You can, for example, use ${#${#}} to disambiguate ${##}. It doesn't remedy any of overloading described in the article, but it does at least make it possible to understand the implied meaning(rainbow highlighting for brackets probably helps here too).

For the most part I'd prefer the syntax suggested in the posted document, but I'm pointing this out as it can be useful as a way to apply arbitrary zsh flags to simple strings to remove the need for external tools like sed(fex, ${(flags)${:-test}:flags}).

Final odd trivia note, you can make the syntax even worse with zsh as # is also an expansion flag when surrounded by braces. ${(#)#:-0x23} is valid -- and woefully unreadable -- syntax to count the length of the "#" string.

¹ https://zsh.sourceforge.io/Doc/Release/Expansion.html#Rules