Lapulapu's forces being 1500+ is a extremely heavy overexaggeration, the censuses from that century show it would only be a small fraction of that number.

And he wasnt stupid for thinking he could win; a small armored force defeating a large force of indigenous tribes isn't that rare, if anything this should have been an easy victory. Lapulapu was just a better tactician, or more so Magellan was a poor tactician.