I interpreted Musk's comment to reflect the fact that rockets generally will be destroyed if the tanks lose pressurization and/or the rocket tumbles. The (load bearing) tank walls usually cannot support major side loads. The Starship stack needs to be a lot stronger than "usual" rockets to withstand the rotational forces and aerodynamic forces involved in landing them.

Ref: https://headedforspace.com/why-and-how-rocket-fuel-tanks-are...

OTOH, most rockets are transported horizontally. (insert picture of Falcon9 going down a road on a flatbed here). This means that they are designed to withstand 1g side loads. SuperHeavy is always roughly vertical. Starship of course has to withstand insane loading horizontally during it's crazy descent and flip stage, but SuperHeavy stays vertical.

The tanks are held in supporting cradles when horizontal so that their weight is distributed properly. Sometimes the tanks must be pressurized when transported horizontally or they would be damaged.