Beyond the obvious (and possibly true) comments to be made about idealists with less exposure to the realities of life, and the general truism that voters in general seem to be more tribal than logical, the fact is that you can support a candidate despite him having one policy you disagree with if you believe that his or her package as a whole is the best. Presumably for many of these this appeals as a package, even with the drawback of Euroscepticism.
There is a further point that labour have gone to great lengths to avoid having any real discernable policy at all on Brexit, and this would allow people to believe that when push comes to shove it will be the cautious approach that would be taken (i.e no Brexit) especially with the latest official position (as of this post being written).