After reversing, we get something like this:

\procedure wikipediaLink(linkText:"Wikipedia Link", articleName)
  <a href=`https://en.wikipedia.org/wiki/$(articleName)$`>
  <<linkText>></a>
\end

<<wikipediaLink Aardvark>>
<<wikipediaLink "Wikipedia on aardvarks" Aardvark>>

In the first link, we can see that articleName has implicitly gotten a value of "" (an empty value), because there is nothing after the wiki/. In fact, the same thing happens if you don't provide any default value at all and just don't include a parameter:

\procedure wikipediaLink(linkText, articleName)
  <a href=`https://en.wikipedia.org/wiki/$(articleName)$`>
  <<linkText>></a>
\end

<<wikipediaLink Aardvark>>

This said, putting non-optional parameters after optional ones, or calling a procedure with some non-optional parameters missing, is bad style and will likely get you very confused sooner or later, so you should certainly avoid it!