I’m uncertain if you tried pedalling an Onyx ebike that had a throttle you didn’t use while riding it, or if you tried peddling some other ebike that had power assist but no throttle control.

Either way, something with pedal assist shouldn’t feel like riding underwater even if the battery is dead. Similar to an escalator becoming stairs in a power failure, an ebike should just become a bike. Though it would be noticeably heavier.

I suppose how well it does unpowered is more an indication of build quality though. Certainly some ebikes are so shoddy they are practically unrideable when the battery is depleted.