The RTI process should be used only for determining whether a child has a specific learning disability and is different than the determination of whether another diagnosed impairment such as autism or ADHD adversely affects educational achievement. ADHD is NOT a specific learning disability, but is clearly classed as an "other health impairment".
Since you brought up RTI, though, under
34 CFR 300.309, a discrepancy between ability and achievement is one of the three allowable ways to determine that a child has a Specific Learning Disability. IDEA 2004 did not say that states could not use this criterion, only that they could not make it a *required* criterion. States are responsible for developing their own criteria for identifying children as children with a disability, and many of them either did not include this method of determination, or else directly prohibit it.
You can read the preamble to the regulations implementing IDEA 2004 and analysis of comments and changes
here. If there is a particular topic in IDEA you are interested in, such as identification of SLD or general eligibility criteria, simply scroll down to the relevant section number (note that there is often discussion of several different points concerning the same section, since the analysis and discussion is based on what sorts of comments were received regarding the regulations.) It's interesting reading.
