In the proposed soft-fork, pre-softfork nodes don't see Alice sending coins to Bob, since they will see each block as *only* containing a coinbase transaction. It will look to them like Bitcoin has become permanently untransactable.
What the proposal shows is that the notion of soft-fork is not as clear cut as it seems at first, and is really more
of a spectrum, based on how much of the new rules are being verified, or even visible, to old nodes.
I think what Peter Todd is trying to demonstrate is that the coinbase can be along with the rest of the transactions in a block, but the coinbase transaction has a new output that is spent in the autogenerated newCoinbase transaction that can be pretty much anything, as long as it's less than the current block reward.
This does not really create tail emission though, it creates temporary tail emission where part of the mining rewards have to be delayed so that they are received at a later block far past the year 2140. It is no longer possible to do once the block reward is completely exhausted.
But that's going to damage the economics of Bitcoin in the present, so I would not support such a fork.