No the hole was drilled so they could drill the oil passage for the pushrod cup to get oil.
The thread and rocker have an interference fix on these rockers. Yes it got harder to turn because you were getting more thread contact as it went in. The Isky rockers in my motor use a lock nut arrangement.
You could tap that hole all the way to the adjuster and then use a set screw to lock the adjuster. If you were having trouble with them backing off.
Moper Performance written 1978 page 32 tells about the oil leaking past the screw an out the hole.