Holley sniper EFI motor suddenly revving uncontrollably when I blip the throttle

I don't know the Holley Sniper, but for throttle bodies in general (both TBI and MPFI), the throttle usually closes off tightly and all air flow at idle is thru the IAC valve. Thus, if you truly plugged the IAC port, the engine should have died, unless getting air somewhere else (vacuum leak from PCV, brake booster, intake gasket). Did you just cover the external hole? If so, that doesn't block the airflow around the throttle plate. You must block the internal hole which the valve plug covers. Speaking of vacuum leaks, my 1996 Plymouth 2.4L began idling way too fast. I figured a leak was overwhelming the IAC authority (full closed). Finally found it by flowing water from a garden hose around the intake. It slowed way down when flowed around the intake to block interface. Hard to see since the intake loops over that. The intake manifold gasket had become brittle and cracked with chunks missing. I had changed it during a head gasket replacement 15 years earlier (MLS gasket to fix a generic oil leak). I recall a name brand like Victor-Reinz but doesn't mean they didn't source them from a Chinese company that used black cardboard.

I had a similar behavior to yours with my 1969 Dart 225 when the Holley 1920 carburetor would run lean at idle (common problem from a plugged idle-metering block). In "P" at idle, if I manually opened the throttle plate briefly and returned it, the engine would speed up and stay idling faster. Shifting to "D", slow the engine way down and would even stall. I suspect the brief rpm increase gave more spark advance, which a lean mixture likes, then it stayed running fast with that more advance. I always thought it idled lean, but 2 rebuilt carbs didn't fix it and no shop could diagnose it (yes, worthless "professionals"). Finally, a 4th carburetor made it purr like a kitten at idle and no more stuttering off the line. Might not relate to your problem, though perhaps general for any lean-running engine. You might be able to diagnose by giving it squirts of Starter Fluid to see if that changes behavior.