Interesting find.
Hardened lead round balls were being used before any ctg. gun was invented.
Even Jacob's, with his 4/winged "hardened bolts" & "explosive bolts" in his 4-grooved SxS used hardened bullet. Tin and Mercury were the usual metals used for hardening up until 1860 or so.
It is possible Gevelot found another method, as in antimony?