That's not how I see it.
In the first place, although we have a fair idea what Labour's negotiating position would be, we do not as yet have much idea what the final deal will be. You appear to be expecting that Corbyn declares now that he will campaign in favour of (or against) an unknown deal. That hardly seems reasonable - and I'm pretty sure that if Corbyn did now declare either way which way he'd campaign, the Conservatives would immediately pounce on that as an attack line.
Secondly, we all saw what happened to David Cameron: Campaigned on one side of the referendum, lost, and instantly found himself in the almost untenable position of having as Prime Minister to implement something that he'd very publicly and very strongly been opposing 2 days before hand.
For those reasons, it seems to me that negotiating the best deal you can, then remaining neutral during the referendum so you can then credibly implement whatever the referendum result is seems to me like quite a sensible strategy. (And it's not like there won't be hundreds of other senior politicians weighing in with their views on either side of the debate).