if() function

Could someone explain why the following code work without compiler errors?

if(
    isnull(ri!user),
    "",
    isusernametaken(ri!user),
    user(ri!user, "firstName") & " " & user(ri!user, "lastName"),
    joinarray(split(ri!user, "."), " ")
)

I expect some kind of evaluation error, but it works just as the three lines starting on "isusernametaken" were an embedded if.

I saw that in one of the Appian courses and I'm wondering if it is some undocumented feature of the if function.

Thank you

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