You couldn't have devised a more definitive test than that. The only other remaining possibility for that battery volt drop would be a starter motor short circuit but I rather expect you'd have seen signs of the magic smoke escaping from the starter or be able to detect evidence of the battery cables getting rather hot.
And that, folks, is the clincher. :-)
Some weird manufacturing defect in the battery. A rather unusual fault but when it comes to electrochemical energy storage, _anything_ is possible.
But only if you don't properly tighten the clamps. Provided you wriggle the clamps a little as you tighten them up to squeeze out the excess grease (vaseline or silicone), you'll get just as good a contact as if it where totally dry. The point of the grease is to stop ingress of any acid into the joint which will corrode a dry joint over time.
I'd presume a good clean _followed_ by the application of vaseline or silicone grease followed by having the clamps properly tightened up.
Lead sulphate corrosion products.
And me, many years ago now. It doesn't seem to be the problem it once was, presumably the terminal post seals are more durable and effective on modern batteries these days.
That's not to suggest you can forego the benefit of a light coating of vaseline over the battery posts and clamps when replacing the battery (after scraping any lead oxide layers off the terminals beforehand) to guard against acid ingress that could arise if the seals are less than perfect or lose their efficacy over the life of the battery which could be as much as ten years.
Grease only becomes an insulator if you allow it to do so. Properly fitted battery clamps precludes this particular side effect quite effectively. What grease resides within the joint will be limited to the microvoids which act as an insulator anyway, the same microvoids that allow any acidic contamination to gain ingress and compromise the initial low resistance contact on first assembly of the joint.
Old hat these days since I've not seen an open vent battery in years. Most are sealed with a small vent tube to one side connected to a pipe to route any (small) fumes to safety.
FWIW, I've never bothered with greasing battery terminals - and never had corrosion either.
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