The UK exports ~50% of its goods and services to the EU. The EU exports 8% of it's good and services to the UK. The latter is in absolute terms the larger amount of goods and services, but the former is just a massive portion of total trade to have to suddenly replace or re-negotiate.
Think about it this way: SMB company A has to suddenly replace 50% of its customer base. Enterprise company B has to suddenly replace 8% of it's customers. I know which one I'd rather be!
Pointing out the reality of the weak and pathetic position the UK finds itself in is not 'scaremongering', it is simply the first of many wake-up calls the UK will be facing in the near future.
They thought they could simply quickly negotiate trade deals using the existing EU ones as a blueprint. I think they now reached a single digit number of agreements but nothing with a huge trade volume (includes economic superpowers like the Faroe Islands or Liechtenstein and as the list goes on it does only get slightly better).
My opinion is that a deal cannot be negotiated among uneven parties. Right now, the UK can either leave with no deal, or leave on the terms demanded by the EU, with no concessions whatsoever. There is no table for negotiation. It is leave or surrender. Once we leave, we are no longer the begging party asking for concessions, but the roles will be reversed. The EU does not want to lose one of their largest export markets, and will quickly change their mind about a trade deal.