That is a matter of interpretation.
I have had a lot of experience with lawyers. I am no legal expert but I have had my share of scrapes. Like everyone else I have to pay for legal representation and I pay $5000 a day for my QC. I have a matter before the courts now that is about to go in my favor as an out of court settlement. A Civil Business Litigation against other entities I was dealing with in a deal that went feril. Law is a very funny business. Law isn't a justice system but a legal system. I've spent a lot of money on lawyers and QCs over the years.
If I was in Boris' shoes, I would not agree or interpret that I would be forced to send such a traitorous letter against The Interests of the UK to the EU and therefore, I would most certainly consider not sending any letter at all, or if I do send the letter, I would accompany the letter with a second letter of SOS and claiming undue coercion and blackmail to ask for something I do not agree with all written and vetted of course by Government Silks who can word the letters in the correct legal lingo.
Also, it is likely that France is now going to veto so if Boris sends a letter it could be because he has made veto arrangements with another EU country like Hungary or France.