He didn't advocate anything. He didn't understand Customs Unions or very much else, had no position and no interest. As a result there was a tug of war on those below him. Milne and most of the LOTO project officers wanted Brexit, not because they wanted to deliver on the vote, but because they always wanted Brexit.
The Party overwhelmingly didn't want Brexit. Once the vote happened many felt it should happen to respect the vote, including Starmer. A number of MP's wanted it in order to save their seat.
However it was the Brexiters that didn't respect the vote, by trying to impose a version of Brexit that had never been presented to the voters. That was when Starmer came up with the Customs Union/Single Market position. It respected the vote but also respected the promises made by the Leave campaigns and on top of everything else.
The reasoned, logical position, as the lack of a plan unfiolded, as the Brexit people were being pushed in to was not necessarily one that they had been promised and as 18 months of polls had shown not one with a majority in favour of Brexit, was to put the actual Brexit deal and future position back to the people to a confirmatory vote.
Shame on anyone who, regardless of how they voted in 2016, did not hold that position and view in 2019.