Technically, she wasn't, and was.
"Treaty cruiser" is a term given to those ships designed to the limits of the 1922 Washington treaty: 10,000 tons and 8" guns. Total numbers and tonnage were not limited
But those ships were limited by the 1930 London treaty. Limits of 8" cruisers were actually 6:5:4. At this stage the US had only built 8, so there were no real restrictions to the building program (the last of the next 10, Wichita, in 1937, as actually a modified Brooklyn class, built to fill out the allocation).
The UK was right on the limit and had to cancel 8 ships authourised in 1927, 1928, and 1929.
Japan was already building the twelvth, but could complete all authorised ships, swiching to to nominal "10,000 tonners" with 15 6.1" guns in the 1931 allocation. This was not what the round-eyed parties expected (The US had less than half her allocation built but really didn't want to spend money building more cruisers during the Depression, and the UK really wanted smaller 6" cruisers built in larger numbers for her strategic needs)
The London treaty also imposed tonnage limits for 6" cruisers - 143,000 tons total for the US - but as at the time the US only had 10 small preWashington Omaha class, techically the first 8 Brooklyns fell within the London treaty limits.
London Naval Treaty limits of 8” cruisers were 18 US, 15 UK, 12 Japan.