RubyMotion doesn't use ARC (or Xcode) at all -- it doesn't produce Objective-C code as its output, but executable bytecode intended to be run on the target. The output of Objective-C and RubyMotion, that is, is the same. So you need an equivalent implementation of ARC in RubyMotion to deal with memory management, unless you want to do it manually, which wouldn't make RubyMotion very Ruby-like at all.
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RubyMotion doesn't use ARC (or Xcode) at all -- it doesn't produce Objective-C code as its output, but executable bytecode intended to be run on the target. The output of Objective-C and RubyMotion, that is, is the same. So you need an equivalent implementation of ARC in RubyMotion to deal with memory management, unless you want to do it manually, which wouldn't make RubyMotion very Ruby-like at all.
Debatably, as is the subject of this article.