You're definitely right that "introduced" opens the door for the fact that the topic may initially come into play in lines 20-23 but be further elaborated or touched upon later as well.
But I would also heed Matt's original comment that an Attitude question is almost always something of a big picture question, so we definitely should expand our sense of "what is this question testing?" to the whole passage.
You will normally still find one (maybe two) sentences that act as your concrete support for an Attitude question, but there's no reason to think that the author will reveal his attitude about something the first time that thing is mentioned in the passage.
Picture a typical LSAT passage in which the 2nd paragraph begins with "Some critics feel that ....". Normally, the author will dedicate a paragraph or two to their point of view before chiming in with his own BUT/YET/HOWEVER "I disagree" paragraph.
If LSAT wanted to ask about the author's attitude regarding those critics, it could say "Which of the following most accurately describes the author's attitude toward the critics mentioned in line 20?"
That line reference is just identifying who/what we're talking about. It's not saying that line 20 is where we'll find the author's attitude.
In this hypothetical case, we wouldn't actually get the author's attitude about these critics until many lines later in some subsequent paragraph.
Generally, question stems with line numbers have correct answers that are supported by a different sentence in the passage (sometimes it's a sentence immediately before or after the line number in the question, sometimes it's far away).
If LSAT wants to guide you to a specific sentence using the question stem, I think they're more likely to just provide keywords than to provide a line reference.
Happy hunting.