I think that's misunderstanding what this is saying. Plants are inputs as feed to chickens. That's the starting point this is using and comparing the losses to

It wouldn't make much sense to, for example, compare sunlight energy as an input

A significant part of most plants we eat is inedible. If we’re counting animal excreta as a loss we should count inedible parts of plants.

It's also completely ignoring the vast distribution supply chain required to get vegetables out of season. Yes, if you only eat food which grows naturally in your area and then preserve it for the winter, it's very nearly zero impact. But if you eat kale delivered by diesel truck 2000 miles away, it's probably worse in terms of carbon footprint than eating some local chicken.

I have seen a handful of studies that suggest electric bikes charged with solar are lower impact than an analog bike due to how dirty the food supply chain in the US is. I kind of suspect being vegan would actually make it worse, simple due to the food being less calorie dense on average, likely resulting in a lower calorie per carbon-mile equivalent.