A hard Brexit doesn't mean the EU will impose an embargo on the UK. The EU now happily exports food to WTO members outside the EU, and doesn't impose export tolls or restrictions on those that I'm aware of.
Of course the UK might impose its own tolls, quotas and other import restrictions after crashing out of the EU, but that'll be its own doing.
Edit (since OP updated): It seems implausible that the government would make that self-imposed process onerous enough to result in a literal famine. Which goes to the heart of what you've misunderstood here.
This "chaos at the border" refers to chaos for UK companies trying to export things due to customs requirements the EU has, whereas the UK's hypothetical inability to import something would be due to a purely self-imposed post-EU customs process.