Castor was using the acronym JDO long before Sun's JSR was public knowledge. I was trying Castor out, and it was from them that I found out about the "beta" JSR (I believe it was versioned 0.8.1 or some such) which had languished in that position for some time (the most current Sun doc's were 6 months old at that time). The fellows at Exolab claim to have attempted to work with the JSR folks to come to some compromise on their architectural differences.
Castor has never made any claims to supporting Sun's JDO, quite the opposite. And there were several O/R tools that touted "java data objects" even before EJB.
Finally, I read the JDO spec (at the 0.8.1 time) and found it just as intrusive to business objects as EJB. The vendor-lock potential was stupendous.