He spoke in meaningless terms such as 'making Brexit work', and 'eliminating border controls' even though that is not within the UK's power. He talked of eliminating red tape when the only red tape we must deal with is that which we have to deal with as a direct result of Brexit.
He will soon be facing a new Conservative prime minister who could well have voted to remain in the EU and who may well wish to be closer to the EU, given the immense damage Brexit has already done to our economy. By making this commitment Starmer has closed off any room to move if public opinion begins to favour closer union with the EU.