Fuel jettison limit weight seems to be a fairly new addition - it popped up in the FCOM some ~2 years ago (at least on the 787).
As you guessed, it's a landing climb limited weight plus fuel burned over 30 minutes and fuel jettisoned over 15 minutes. So basically it is a takeoff limiting weight assuming the scenario that you turn back to the departure airport and land after 30 minutes, having dumped fuel for 15 minutes.
I don't know if it is some new certification requirement - haven't found anything in he EASA OPS rules. Perhaps it's somewhere in CS-25. Another thing I don't understand is that Boeing mentions ZFW as a factor here. The tables in the FCOM are based on maximum certified MZFW, however Mr. Boeing says that using lower ZFW may yield less restrictive performance...
EDIT:
I have found an exhaustive paper on this new requirement by the FAA