I see a lot of ! there (following your link), can you point me to exactly which part we're talking about?

I see a lot of ! there (following your link), can you point me to exactly which part we're talking about?

This is the relevant line:

!define !defineifexist "!insertmacro !defineifexist"

It's basically a shortcut that assigns the !insertmacro instruction to a definition. The following two do the same:

# Default !insertmacro syntax
!insertmacro !defineexists [params]

# Shortcut for !insertmacro
${!defineexist} [params]

PS: Unlike variables, defines can begin with any allowed characters – including ! and .

Since I think the syntax in the linked example is kind of terrible, here's a version that only uses ! where necessary:

!macro DefineIfExist _VAR_NAME _FILE_NAME
	# macro code omitted for brevity
!macroend
!define !defineifexist "!insertmacro DefineIfExist"

In NSIS, all compile-time commands begin with a !. I guess the author's intention to name the definition as !defineifexists was to reflect that his macro does compile-time operations. It could be named anything.