I think you missed the point. The article makes no mention of how existing implementations handle this case. It seems the author had only theorized based on his knowledge of the TCP protocol.
I think that if an implementation treats it in either of the following ways, then it's good: 1) treat an ahead-of-sequence ack as "everything acknowledged so far" or, 2) drop the ahead-of-sequence ack as a bad frame.
Other behaviors, like crashing or messing up the stream, of course, spoil things.
There is a problem if the receiver sends only an ahead-of-sequence ack, without acknowledging frames before, and the sender drops that ack. The sender must acknowledge everything actually received, and respond properly to window probes, to ensure forward progress.
I think you missed the point. The article makes no mention of how existing implementations handle this case. It seems the author had only theorized based on his knowledge of the TCP protocol.