Why is this not the sea-lion pool with a definite article instead of a sea-lion pool with an indefinite article?
And the decision to build the zoo around a sea-lion pool was the crowning touch.
It is a particular pool, and it has been mentioned in the text before, so isn’t that the wrong kind of article to use here?
Both 'the' and 'a' are fine, but express different things.
When using 'a pool' it implies that at the time of the decision there was no specific sea-lion pool to talk about. It hints they did not decide for this particular pool, but any pool. Maybe this particular pool was designed later by the architect.
When using 'the pool' it would imply a decision to build this particular design for the pool.
Caro invites you figuratively into the planning—you are to imagine Robert Moses saying at that time "We're going to build this around a sea-lion pool". When the decision was reached the pool didn't exist.
"The specific pool" that is "referred to in the text" did not exist until after it had been built.
The OP's sentence is talking about the not yet well-defined concept of "A pool" at the time when the zoo was being designed.
There is nothing wrong with changing the sentence to "the pool," but it tends to suggest that the speaker had some definite (detailed) ideas about what the pool was going to be (its size, shape, etc) rather than the indefinite (vague) idea that "the main feature of the zoo is going to be a pool".
The difference between "a" and "the" is that the sentence "We are going to build the zoo around the sea-lion pool" implies that the pool already existed and had been chosen as a site to build the zoo.
Whereas the sentence "we are going to build the zoo around a sea-lion pool" implies that no pool currently exists and that the structure and design of the zoo will feature a sea-lion pool that will be built as a centrepiece.
