This pdf seems to hold at least part of the answer.
It shows an OLT with 14 line cards; each line card has four singlemode fibre output ports. That makes a total of 56 singlemode fibres per OLT chassis.
Each fibre can be split into 32 fibres at a splitter. Each of these latter fibres goes to a customer’s ONT, supporting a total of 1792 customers (or subscriptions, at least). This matches the “thousands” referred to in the article.
What this does not answer is the context of the article’s author’s reference to 128 customers per OLT.
The other question, regarding the OLT density in a central office, would have an interesting answer but the answer is most likely going to refer to a lot of discretion in the hands of the service’s architects.