This analysis does not account for side benefit of the oxygen. If you split water to get hydrogen, then for every kilogram of hydrogen you get, you also get 8 kg of oxygen. Liquid oxygen is not an expensive commodity, its market price is about $1/kg, but in this context this makes a difference. For example, in the first infographic, the cost of green hydrogen produced today is listed as £16.97 which is about $23. If you can recoup $8 from this by selling the oxygen, or even only $5, then this makes a difference. If you select green H2 with 2030 assumptions, you get £7.67, or about $10s. If you sell the oxygen at $5, you basically get the hydrogen at half price, and this makes the hydrogen powered truck slightly more economical than the battery powered one.

The current market price is based on current supply and demand. Splitting water to create enough hydrogen for non-trivial fraction of the transportation sector would generate an enormous amount of oxygen. The price of oxygen would likely tank in that situation.

Does the cost of green hydrogen not already price this in? It would be crazy to go through the trouble of electrolysis and just vent the oxygen into the atmosphere

Except you can make oxygen for pretty cheap using oxygen concentrators. The technology is simple enough that home versions exist for patients with lung problems can lug one around at all times to have a feed of oxygen rich air. Oxygen is almost 21% of the air we breathe, it's trivial to capture. Hydrogen counts for only 0.000055%.