There is also tyre wear particles to be considered! /s

Just gotta eat seaweed like those cows

Hmm, thinking about it, I think maybe the direct CO2 exhaled during exercise may not be the most useful metric for human-powered travel. Every atom of that carbon was recently removed from the atmosphere by the plants you ate or that went to feed the animals you ate. It isn't carbon that was underground for millions of years as is the case with fossil fuels.

Unfortunately, growing the food does involve carbon emissions from fossil fuels. Taking this page's number of 2.5t/yr for the typical American diet which they assume to be 2,600 kcal/day that works out to 2.6g of CO2 / kcal (2.5t / 365 / 2600 = 2.6E-6 t = 2.6g), or 52.7 g / km for cycling, or similar to an electric car if the chart is at all comparable (I don't know the chart's methodology; for example for the fossil fuel transport options does it count the carbon cost of producing and transporting the fuel or just the tailpipe emissions?). Changing one's diet looks like it would improve this; the best-case would be a vegan diet which would result in 31.6 g / km.

Now that's just based on numbers from that one source, so I don't know how reliable they are. It does say it includes the large amount of wasted food in the final number, and I don't know if the numbers in the original chart are that level of conscientious. Regardless I think the takeaway here isn't that cycling is bad, it's that our food production system is terrible and it badly needs to become way less carbon-intensive.