There is no precedent I am aware of for a British government signing up to an international treaty it cannot withdraw from without the agreement of other countries.
I'm confused. Isn't it quite usual that international treaties are binding on the signing parties? To take a particularly relevant example, the Good Friday Agreement is an international peace treaty of which the United Kingdom is a party. It is a legal agreement, not just a political intention. Swiss voters recently rejected a referendum to give Swiss law precedence over international law; wouldn't the reverse (the status quo) mean that indeed, when a country signs up to an international agreement, this is legally binding? And doesn't legally binding imply that a party can't legally unilaterally withdraw from it?
Is Corbyn wrong / unaware, or is there indeed something particularly unusual about the binding nature of the backstop in the proposed Brexit agreement — compared to other international treaties of which the United Kingdom is a signatory?