You're right - technically, anything that produces a trade signal is an indicator. Therefore the manual distiguishes between the classical indicators used in traditional TA, and indicators that are based on scientific methods such as signal theory or statistics.

This is no sharp distinction though. I think a profitable system can be done even with classical indicators when you identify an inefficiency in a market and use a system of indicators that exploit that inefficiency. This would probably not work with a single indicator, but with a more complex combination.

In the book by Tomasini/Jaekle, I'd draw the border between the Luxor and Bollinger band systems that are based on classical indicators and are not really recommended for trading, and the triangle system that is based on the observation of an inefficiency and thus might have an edge.