Not since SBP it isn't. It got a 33% reduction to its main gun damage.
Upgraded, it's about on par with the Luchs now. Greyhound vs Luchs is a pretty fair fight.
Without the .50 cal upgrade, it's got a solid chance to lose against a 222.
Mirage means that USF's other light vehicles are balanced in their single tier unlock timings, as the M5 (Stuart) is mainly anti-vehicle and the other ones have low health. If the M8 would come at similar timing as M5/AA HT/M20, but with great AI performance and more HP, it would be OP. He wasn't talking about the M8 Greyhound in the sentence you quoted, but about the M5 Stuart.