If I'm understanding this proposal correctly, it could be described as a hard fork with a simultaneous majority hash power attack on the old fork.
The OP has indicated a method by which the hash power that hashes the blocks on the new format side of the hard fork can simultaneously hash empty blocks on the old format side.
That way, once the hashing participants on the new format side have enough hash power, the old format side will no longer be able to confirm any transactions ever again. They will be forced to either hard fork to keep their old format, or they will have to join the new format.
So it really would just be discouraging any use of the old chain and force people to switch to the new one.
The way I understood it was that it is supposed to be that people on the old chain can still use Bitcoin normally. As I said in an earlier post, this is not possible unless somehow all of the UTXOs in the new block are presented in the old block format. But I think in order to do that, a software upgrade is still needed so that the client actually knows what to do with that data because it cannot be in the same format that is currently used. Either way, I don't think the idea that this is a soft fork works, it is still a hard fork.