You mean the Me-163, He-176, X-1, X-1A, X-2, X-4, and X-15 weren't
rockets, they were jets? Someone call the Air Force!
The X-1 had a horizontal rocket-fuel tank stressed for 20 G's. The Air Force has hung iiquid fueled intercept missiles on aircraft, where they experience severe sideward G-loads as a part of normal operations. Apparently, although obviously nobody in rocketry today has the knowledge of how to do it, at one time engineers knew how to design a horizontal fuel cylinder to a set of design specifications. (Some engineers in fields like conventional aviation, rail and truck transport, and the energy industries have hidden away these design secrets). Elon Musk must've recruited on of these engineers to check the Falcon 4 for Stratolaunch operations, probably using some old, yellowed, classified document on metal ribs and stringers captured from Peenemunde and kept in a locked vault next to the Arc of the Covenant.
There's dense, and then there's black-hole dense. If an Air Force general got a bee in his bonnet and decided his new ballistic missile should be stored horizontally for easier mobile deployment, leap in the air horizontally, and then transition to vertical flight to cut several minutes off the response time, the aerospace contractors would just ask "How high before it pivots?" It's not a difficult problem, and you can do it just fine with solids (which, after all, allow for precision or we wouldn't be using them for ICBM's or the Space Shuttle), especially considering that the only requirement is to finish the mauenver pointing up, with an allowable error meaured in tens of degrees, just like an SLBM before main stage ignition. To the military, this kind of thing is trivial. To NASA types it's apparently more baffling than a warp drive and they think it simply can't be done.