It's a (slightly exaggerated) reference to the Irish backstop.

The UK, as a sovereign nation, is party to international agreements which it has signed in the past which commit it to doing various things, many of them intended to be permanent and many unrelated to the EU. One of those is with the Republic of Ireland, over peaceful relations in the UK territory of Northern Ireland. The Irish government and many Northern Irish people regard this agreement, broadly construed, as implying that no new barriers will be set up on the Irish border. Another international agreement, WTO rules, which the EU is signed up to (and the UK also intends to trade under) implies that if the UK and EU adopt different customs regimes they will also be required to actively monitor customs borders between the UK trading bloc and the EU trading bloc. The Irish government (and by an extension EU negotiators representing their interests) regards feasible customs border check regimes as violating the spirit of the Good Friday agreement over security barriers. Therefore unless and until some border solution in line with WTO regulations and acceptable to the Irish government or an adjudicating party is found (or the Republic of Ireland also chooses to leave the EU, which is highly unlikely) Northern Ireland must not become part of a different customs area from the Republic of Ireland and therefore must remain subject to some relevant EU laws.

Being a sovereign country means the possibility of reneging upon prior commitments, but it doesn't mean these agreements don't exist or that parties won't insist you honour [their interpretation of] them when offering new agreements