Good battery or not if a drain was on it, it could take it down and not start. When charged and known charged it shoud test well and start/crank the engine with expected power as you have noticed when jumping it.
Charging system may not be maintaining battery at full charge or drain (mirror thing unknown as to whether that could do that) it still points to battery new or not.
I don't care if the car has a charging system, if a known charged up battery can't start it and a jump makes it start normally the problem is the battery or the connections right at it - same as first said.
GM battery cable ends at the battery are iffy when they get older. Take a hard look at those. When jumping the side post type you are connecting to the battery and still relying on the connection of the bolt. The "U" bolt type jumping is direct to the cable first, in fact with good jumper cables you don't even need a battery in the car. The GM side posts are hard to jump unless bolt thru cable end has strong connection there which is using the battery as the "nut" if you will. If a battery is a shorted mess for example jumping those is a trick vs U bolt type,
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