It's not a fugue-within-a-fugue texture. But it is a fugal exposition within a larger fugal exposition, both in terms of harmony and phrasing. That's important because it is the larger exposition which sets the pacing for the following sections of that opening movement.
It's also important from the standpoint of musical drama. Imagine someone tells you a story and gives you the essential setting in the first 17 seconds. Then over the next 10 seconds they begin to describe what sounds like the story's rising drama. Now you have an expectation for the form of this short story, and you start to listen more intently.
However, they then begin laying out another, closely related setting followed by its own little internal conflict. Finally this leads to a description of a setting that sounds like an intensified and more detailed version of the original setting. Only then do they finish the actual exposition of their story and move into the rising action.
At some point in all of that you're going to get the sense that this is a much more involved story than you had initially anticipated. You're also going to anticipate that the rising action itself is going to last a lot longer than 17 seconds.
That's a fitting dramatic arc for introducing the listener to the subject matter of St. Matthew Passion.
Also, to better answer the question about listening vs. analyzing notes: I just sang the opening melodies to myself to get the durations for the analogy above. I originally got the idea about the exposition-of-expositions walking somewhere and being struck by the relative brevity of the fugue subject compared to the gravity of that movement. (As well as the fact that the entire inner exposition reappears twice later in the piece, which AFAICT never happens in any of the other music Bach's wrote.)
There is of course a long-standing debate over music for the ears vs. music for the eyes. But I don't think this falls into that at all-- everything I'm describing can be heard and weighed with a musician's ear in the same way an experienced coder can make an assessment that some code "smells", etc.
It would be more accurate to say that you find it reminiscent of a fugual answer, or something along those lines. Which would be fair enough as an observation.
It would be less accurate to say that. It's a subject-answer-subject fugal exposition which itself consists of three smaller subject-answer-subject fugal expositions. Both are fugal expositions in terms of the harmonic progression and the melodic repetitions.
If Bach just repeated the fugal exposition again on the dominant, it would be a different piece.
It's also important from the standpoint of musical drama. Imagine someone tells you a story and gives you the essential setting in the first 17 seconds. Then over the next 10 seconds they begin to describe what sounds like the story's rising drama. Now you have an expectation for the form of this short story, and you start to listen more intently.
However, they then begin laying out another, closely related setting followed by its own little internal conflict. Finally this leads to a description of a setting that sounds like an intensified and more detailed version of the original setting. Only then do they finish the actual exposition of their story and move into the rising action.
At some point in all of that you're going to get the sense that this is a much more involved story than you had initially anticipated. You're also going to anticipate that the rising action itself is going to last a lot longer than 17 seconds.
That's a fitting dramatic arc for introducing the listener to the subject matter of St. Matthew Passion.
Also, to better answer the question about listening vs. analyzing notes: I just sang the opening melodies to myself to get the durations for the analogy above. I originally got the idea about the exposition-of-expositions walking somewhere and being struck by the relative brevity of the fugue subject compared to the gravity of that movement. (As well as the fact that the entire inner exposition reappears twice later in the piece, which AFAICT never happens in any of the other music Bach's wrote.)
There is of course a long-standing debate over music for the ears vs. music for the eyes. But I don't think this falls into that at all-- everything I'm describing can be heard and weighed with a musician's ear in the same way an experienced coder can make an assessment that some code "smells", etc.