Your theory about thermal expansion is mostly correct. Wood is not dimensionally stable when exposed to weather, as it expands and contracts with heat and humidity it can cause a coating to break the bond and fail. Once a small failure occurs moisture enters and accelerates the process because the moisture will cause pressure as it warms in the sun and tries to escape the wood. This will cause lifting and cracking of the finish.
Spar varnishes are flexible and can expand and contract with the wood reducing the possibility of failure. However, eventually UV and weather takes its toll and the finish fails. A common point of failure is a sharp corner. The finish is thinnest at that point and subject to wear allowing it to fail. If you notice the cap rails on @D'Arcy's and
@Tom Y's boats are rounded preventing this thin spot. Sometimes aesthetics are actually functional.
Epoxy, even penetrating epoxy does not chemically bond with the wood. Epoxy is a good adhesive and it attaches to and saturates the pores in the wood. This saturating the wood effectively increases the surface area to which the epoxy can bond. There is nothing special about penetrating epoxy, plain old West System can penetrate the wood and seal it. "Penetrating Epoxies" are low viscosity, so are all the other epoxies when they are warm enough. West System at a very warm room temp ~75+° will saturate wood. Warming the wood prior to applying the epoxy will cause more epoxy to be absorbed into the wood grain. Warming reduces the moisture in the wood and reduces the air pressure. Once the epoxy is applied and the wood begins to cool it forms a low pressure vacuum and the epoxy flows deeper into the grain. Sort of like vacuum bagging.
Once applied on brightwork, the epoxy must be protected by multiple coats of UV protection. If the varnish is worn UV will damage the epoxy and the varnish and the epoxy will fail.
Your marina mates are working too hard. If applied correctly with a sufficient number of initial coats, all Cetol needs is a quick cleaning and light sanding before applying another quick coat. Do a little work each year or a lot of work every few years, either way it adds up to about the same.