You overlooked the much more likely third option: I don't believe these claims are true. Most people supporting no deal don't believe they're true, and for valid reasons!

Let's break it down.

Loss of hundreds of thousands of jobs. This is an economic prediction so we can't know for sure if it would be true or not unless it really happens. But before the vote, this very same prediction was made in very similar form and turned out to be completely false.

https://www.gov.uk/government/news/britain-to-enter-recessio...

Britain’s economy would be tipped into a year-long recession, with at least 500,000 jobs lost and GDP around 3.6% lower, following a vote to leave the EU, new Treasury analysis launched today by the Prime Minister and Chancellor shows.

NB: This analysis was about voting to Leave, not actually leaving, so we can test its correctness now. The analysis talks about the uncertainty created by two years of negotiations, saying it would destroy at best 500,000 jobs and at worst 800,000 jobs. Leave won, there have been 3.5 years of uncertainty and the economy is performing incredibly well. Employment has grown strongly since the vote. This prediction posed as scientific fact, but was riven with huge Remain bias and turned out to be nonsense.

So now the same people are making the same claim again, but won't explain why this time they're right or what they learned from their previous failure. It's entirely reasonable to assume they've learned nothing, are making these claims for political reasons and will be wrong again.

OK, now, colossal disruption. Possible interruptions to food and medical supplies.

Really?

The explanation for why such disruption would happen goes like this. The UK will implement more customs checks, which would be slow and temporarily overwhelm customs infrastructure, leading to supply chain disruptions and disasters.

Two problems with this claim.

Firstly, why would any sane government insist on customs checks if people were dying due to lack of medicine or food? Tariff income isn't that important and on day one the precise rules will be the same anyway. It'd be much more sensible to just relax customs checks to whatever level was needed to ensure continuity of supply until enough infrastructure was built.

Secondly and more importantly, people who actually run ports are saying the infrastructure is already built and it's not true that there'd be large disruptions in case of no deal.

https://www.telegraph.co.uk/opinion/2018/12/08/lettersthe-uk...

The CEO of the UK Major Ports Group says:

I write on behalf of the United Kingdom’s major port operators, responsible for handling 75 per cent of the country’s seaborne trade. Dover, handling around 6 per cent of UK port volumes, faces a unique combination of Brexit risk factors that are not faced by most major UK ports. These ports already have the capacity and infrastructure to handle large volumes of both EU and non-EU trade today without logjam.

So apparently the people running the ports think only Dover might have issues, but if it gets overloaded traffic would just go elsewhere. What does the head of Dover Port think about this?

In Bloomberg:

https://www.bloomberg.com/news/articles/2019-09-19/u-k-port-...

The Port of Dover -- through which a sixth of the U.K.’s trade in goods flows -- can cope with any disruption thrown up by a no-deal Brexit, Chief Executive Officer Doug Bannister said, suggesting some of the direr predictions of chaos are wide of the mark.

So people who run ports are saying this is just politically driven scaremongering.

As for medicines, if you look at the NHS advice page, it says the extent of their plans is hiring additional space on planes or ferries to handle temporary transport disruption (which, remember, port operators are saying won't actually be an issue): https://www.england.nhs.uk/eu-exit/medicines/medicines-faq/

So the claims being made here to try and terrify the British population into giving up on Brexit, it's all stuff we heard before or people outside of the establishment are saying isn't true.