Google Spot Instances (preemptibles) are 80% off and that's it. It's simple.
In AWS you simply tell how much you're willing to pay to keep the instance uninterrupted. If there's someone who is willing to pay more, they will get the instance from you and yours will be shut down.
By comparison, if you are able to create a preemptible VM you are guaranteed to pay .2x retail and then likely (but not guaranteed) to have 24h to do your work.
tl;dr: A maximum bid of .2x retail isn't equivalent to preemptible. There's almost certainly a bid for a given instance for a given runtime that would result in the same price, but it varies over time, space, runtime and instances shape.
Disclosure: I work on Google Cloud (and launched preemptible VMs)
In my several years experience using GCE it is indeed extremely likely. Two years ago it wasn't very likely, but it became very likely about a year ago. Incidentally, I'm the guy that convinced Drew Sutherland of this article to use GCE in the first place :-).
I think AWS might be cheaper in this regard but less predictable, so it is a tradeoff.
I haven't looked yet into "Spot Fleets".
Then it is definitely interesting :)