Well, lead is softer still but more massive nonetheless.
To be fair, nickel is indeed reported by several sources as having a higher specific weight than stainless steel (or, to be precise, than the weight range of various SS alloys):
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Metals and Alloys - Densities
Mass, Weight, Density or Specific Gravity of Different Metals
Would that alone account for the difference in tension between the new strings vs. the round core Boomers, without any need to assume a larger core? I have no idea. (But my hunch is no, if the nickel one is only the outer wrap.) And neither do I know if "same number of wraps" means that they managed to use more than one on the G string, which Mr. Moody once said to be difficult to obtain (but he didn't say impossible, IIRC). Also, how does the number of wraps on the G string correlate with this core debate? are the wraps much thinner than is usual? Again, beats me.