You did fully read that article you linked?

It states that repeatedly during the leadership contest when asked he stated that he would vote to leave.

After winning:

“Labour is clear that we should remain in the EU. But we too want to see reform,” he wrote in the Financial Times.

NOT “I believe we should remain in the EU”.

He has also stressed he is “not on the same side of the argument” as David Cameron, despite both leaders backing continued EU membership.

Because Labour wasn’t in power. It needs that meme:

You support remain because you want unfettered access to Free Market Economics

I support remain because it ties Tory hands together on workers rights

We are not the same.