I think it's the wording that gets confusing. A indicator like the RSI is showing a change in momentum not actual trend change, and this often happens before a support or resistance level is hit.
So, it can be said to be "leading" because momentum change proceeds trend change.Ignored
But I still don't think it is a leading indicator. If we look at economic leading indicators, we can see that they really lead the economy. They are not a derivative of gdp for example. Take building permits, inflation or unemployment rate. Pretty much all of them are leading the economic activity without being a derivative of it.
I guess I just have a problem with the notion that a derivative of something should be leading the underlier itself. Of course I know that derivatives can show changes of momentum in the LAST few bars, but that does in no way imply that it is showing the future or leading the underlying instrument.