This is not a bug with the code, as it is a weird design decision. Currently, Squids can spawn anywhere as long as the environment meets the requirements: Still water, between block height 45 and the water level. This means that squids could spawn in your pool, if it was eligible for spawning squids.

To counteract most unwanted spawns, I'd suggest another requirement for spawning squids: Let them only spawn in river-, sea- and deep sea biomes.