There is no chain of title from the depositor to the current owner of a coin.  MtGox being off blockchain and pooled funds makes sure of that. You can trace a particular "coin" back to MtGox, but the trace stops there.  You can't trace back further than that.

The depositors didn't have any "coins" directly and thus have no claim with the actual thief (if one exists).  MtGox had its coins stolen (or embezzled).  The depositors traded Bitcoins for IOUs payable by MtGox.  Much like a bank there is no individual depositor's money.   The bank has a liability to the depositor.  If a bank is robbed it is the bank's money that is being stolen.  Now in the US we have deposit insurance but even if we didn't the bank couldn't just say "sorry that was your particular $100 bill stolen is last weeks robbery).  The banks funds were stolen and the liability the bank owes the depositor still exists.  Now in this case when MtGox lost their coins, they lost the ability to repay those IOUs.  In the size of the theft was smaller MtGox couldn't just say "sorry that was your coins stolen" the liability (IOU) would remain and depositors could seek damages against MtGox.  I know we went a long way round but while MtGox may have a claim against any coins that can be traced back to MtGox, no depositor would.

Still the application of demo dat rule is not a forgone conclusion when it comes to bitcoins.  It doesn't apply to legal tender and it also doesn't apply to bearer instruments (i.e. casino chip), or negotiable instruments (i.e. a check).  It is at least plausible a judge would rule that bitcoins are more like those exceptions than real property.  Until we see a court case we won't know for sure.  I am not a judge so what I think matters little but a bitcoin has more in common with a casino chip (a bearer instrument) then it does with a car (which is a unique specific piece of real property).

Note: to be clear I am not saying I have the answer just pointing out the law is pretty slow to react to technological changes.  This and countless other questions will eventually be decided but the timescale is measured in decades.