TVS Diode Part number PESD24VS1UB,115 lists a 23kV ESD rating in terms of the IEC 6100-4-2 standard, but only a 10 kV for the HBM MIL-STD883 standard.
I am under the assumption the IEC rating is a more stringent rating then the HBM model
How can it be rated higher on this model vs the HBM model?
Is it simply that the part series was only tested up to 10 kV in regard to the HBM, because you typically only care to protect up to 8kV by the normal HBM ratings if I remember correctly?
Table 7 simply calls out the standards, that makes sense
I assume every value given in Table 6 with the expecation of the last row was found when testing to the IEC 61000-4-2 standard.
The real question here:
Is it safe to assume that if a part passes 30kV in a test done to the IEC 61000-4-2 standard will it pass a 25kV test done to the HBM MIL-STD883 standard?
I think that you are mixing up "IEC rating" with "Machine Model" (MM). Yes, the MM rating is more stringent than the HBM one, however it is not the case here, the datasheet does not provide the MM rating.
Table 7 of the datasheet provides requirements of the standards. Table 6 shows ESD specifications of the PESDxS1UB, as you can see the data exceed requirements of the standards.