Minecraft - Why am I not catching any Treasure Items?

I followed the instructions on the MC wiki, namely:

In order to catch items in the treasure category, the bobber must be in open water. This is technically defined as the 5×4×5 vicinity around the bobber (2 blocks away horizontally, 2 blocks above the water surface, and 2 blocks deep). Each horizontal layer in this area must consist only of air and lily pads or water source blocks, waterlogged blocks without collision (such as signs or coral), and bubble columns. These conditions are checked every tick to determine if the player is eligible to receive treasure from a fishing attempt. If the conditions are not met, only items in the fish and junk categories can be obtained from fishing.

This is my fishing setup:

As you can see, the two layers below the water surface are pure water, 2 layers above are pure air and the shores (or any other blocks) are more than 2 blocks away from the bobber. I've also tried removing the boat just in case, even though it was out of range of the supposed 5x4x5 area.

I haven't been able to catch any treasure items after a long time trying (I've caught dozens of junk items). Is the info on the wiki wrong or am I understanding it incorrectly?

Check if all the water blocks are source blocks. If any of them are flowing water, fishing will not yield any treasure items. You are likely to have hard to detect flowing water blocks if you dug out the fishing area by hand (as opposed to finding one that generated naturally).

An easy way to convert downward flowing blocks to source blocks is to plant kelp across the entire bottom layer of your fishing area. When the kelp is placed and grows it will convert all the blocks it occupies to source blocks.

I spent hours trying to make this work and planting the kelp made it work for me.