Afaik it's not the author's intention, rather there's limited options on how to determine district placement in the base game, so they've had to adopt the district placement from the aqueduct.

It's explained in the guide:

Design Tenets: District Placement

Lastly I’ll quickly discuss the placement requirements for the Borough Districts

Placement Requirements are the exact same as the Aqueduct

• Must be adjacent to a city center
• Must be adjacent to a River, Lake, Mountain, or Oasis (Source of freshwater)

This was not an intentional design choice. It was done because it is, unfortunately, the only way I've been able to find to force a district to be built adjacent to the city center. HOWEVER the more I’ve played with it, the more I’ve actually come to appreciate it from a strategy perspective. It makes planning out an urban city with its districts very interesting, and makes you look for different settlement locations than you might have originally. Also, from a realism standpoint it does make some sense since almost every big city on the planet is built adjacent to a significant River, Lake, or Mountain. Should the time come when Firaxis adds the ability to better specify district placement requirements (like those that exist for wonders), this will likely be updated.

That sounds like an issue with the TSL map. Coastal cities in real life still need access to fresh water; looking at pictures of my personal AU favorite Gold Coast, you'll see there's all kinds of rivers and lagoons and stuff right on the other side.

Gold Coast resident here, turns out we have more man made canals than Venice, the city most famous for man made canals. If you get drunk and go swimming in them at night, you WILL get bitten by a Bull Shark, and this is a problem because drunk 2am swimming is the second most popular sport after bin cricket. We could do something about this, but due to the overtly hostile nature of the Australian ecosystem, maintaining the shark population in our canals is cheaper and safer than paying people to remove the crocodiles that would move in without them here and while Bull Sharks require perfect conditions (Zero visibility, mental impairment.) to prey on humans, Crocodiles are apex predators that have existed pretty much unchanged since before the extinction event that killed the dinosaurs and if a camper goes missing in the wilderness, chances are all that's left are the dragged finger marks in the sand between their tent and the billabong they mistakenly camped beside.

Also, I have been enjoying the mod.

Australia doesn't have rivers? Are you stupid?

Originally posted by Chappie:

Australia doesn't have rivers? Are you stupid?

It does, but most of it's major cities are coastal and built on old colonial ports.

All of Australia's cities with 1m+ population were built on river mouths

I'm actually kinda crestfallen about this, as I believe this is the reason that I can't build boroughs adjacent to GREAT rivers a la Real rivers mod.

I guess since CL still counts the great river tiles as "coast" , it refuses to allow me to build boroughs on adjacent tiles... Which REALLY sucks cus the great river COUNTS as fresh water.

I want a this mod but only the stetic of city and expansion progretion of districts and cities. maybe someone make will posible. :)