Agreed.  It's an analysis about a specific scheme.  The paper assumes that miners build subchains according to that scheme, similar to how they presently build the blockchain according to the underlying "Bitcoin scheme."  Miners would benefit by agreeing to that scheme because it would allow them to reduce network orphaning rates and more quickly verify transactions for users.  The scheme is also a Nash equilibrium, in that no "profitable deviation" from the agreed-upon behaviour exists that a single miner could exploit to earn a greater profit.¹

Taken to the academic limit of infinitely-fast weak block generation, the fee market still exists with the subchain scheme.  This was an interesting result that surprised me--I initially suspected the cost of block space might fall to zero (in only the strictly academic case).

The reason you don't understand subchains is the same reason you don't understand the Bitcoin consensus and mining process.  Bitcoin doesn't "work" just because of the rules encoded in the software--it works because there are people who choose which rules to enforce and what behaviour to encourage.  The miners and nodes are not automata; the will not agree to follow the rules that they've deemed bad for the health of the system, just like how they are presently tearing down the wall at 1 MB despite the feverish cries of Blockstream Core devs.

Anyways, if your fixated on schemes that completely eliminate block size dependent orphaning risks, it's easy to come up them.  I mentioned one early in the thread in response to Jonald (i.e., miners agree to mine a "practice" blockchain first such that the real blockchain is a verbatim copy [minus the blockheaders of course] but lagging by 1 hour).  Another scheme would be just to transmit the block headers and tell everyone else to "figure it out from the Merkle root!"  The challenge is to come up with one that may actually be followed.

¹This is why my analysis assumed miners built off the "highest fee" subchain rather than the "most work" subchain.  There would be a profitable deviation away from the most-work subchain, even though I suspect that is the behaviour that most miners would actually adhere to.