This is absurd reasoning.

All the UK will be doing is following the treaties in place to end an existing agreement. That's not breaking a treaty - it is following it.

When that treaty is merely "you cannot sign trade deals" I don't think people signing trade deals would care. That rule exists for the EU's benefit, not for the benefit of member states.

Thats not the point of deals - merely convincing your prospective trade partner that a deal is unfair to you at that time is a hard sell.

The UK signed the deal knowing the consequences of it, why would another country sign a trade deal with you knowing that you will not follow the terms due to your countries internal politics ?

Because that's always a risk with any deal. Look at how many international treaties the USA violates. People still sign deals with them.