They said "drill and tap". It's tapping the hole for the screws that's the problem. They don't want you to use the mount, itself. to retain the screws. Drill for bolts and use nylon locknuts. Put a washer under the head of the bolt, and one between the nut and the mount. If you tapped the mount and properly-tightened the bolts, then you've also put a strain on the mount. This would also go for drilling slightly undersized and using sheet-metal screws. Another no-no. That effectively does the same thing as tapping the mount.