He could not even sound convincingly pro-EU during referendum campaign when he had to pretend to adhere to this New Labour policy (and probably cost referendum votes as a result), when his position in Labour party was a bit precarious.
Why would he change his convictions now, when his position as a leader in Labour is much stronger?
New Labour is dead. Being pro-EU was Tony Blair’s policy, not traditional Labour policy. In 1970s it was Tories who were pro-EU while Labour were against. With new Old Labour we just see the return to the old policies.