Your question is a ominous mixture of multiple misconceptions about expl3.
First of all, the
expl3.pdf document is the design document of
expl3 which explains the rationale behind the architecture and how to use it. By virtue of its purpose
expl3.pdf of course does not discuss control structures, which is the task of the reference manual
expl3 does not supersede the
ifthen package, just like apples do not supersede oranges. They are complementary approaches to the same problem. Therefore they should also not be mixed unless you understand the implications.
Finally you should avoid mixing TikZ code and
expl3, again because of clashes in the design philosophy. One notorious gotcha in the TeX world is the “spurious space” problem, which
expl3 solves by ignoring all whitespace within
\ExplSyntaxOn...Off. However, this is incompatible with any other code that relies on the existence of whitespace, such as
pgfkeys which form the fundamentals of TikZ. Nevertheless it is possible to make the two work together reasonably well, but this requires the user to write appropriate wrappers which join the space-less and the space-ful world.