But how can they rule out the water leaving the planet very quickly after a long time of Earth Like conditions?

It's been a while since I took a solar system geology course but IIRC Venus undergoes periodic planet wide resurfacing events that wipe out the geological record.

I'm not sure if we can tell when those events started happening. It's entirely plausible they only started happening recently (geologically speaking) which possibly would not show up in the gas analysis.

Again, I'm no expert.