I would say ARM have expertise in 'designing for manufacture' which is distinct from the actual manufacturing, but lets say they have no expertise in 'production manufacturing'.
While Samsung does indeed contain a bunch of largely separate companies, both chips and phones are part of Samsung Electronics, there is one set of top management for both.
They have most likely done many calculations that clearly say "if we lose Apple on the supply side, we lose <insert ridiculous amount of money that eclipses smartphone revenue here>."
While Samsung does indeed contain a bunch of largely separate companies, both chips and phones are part of Samsung Electronics, there is one set of top management for both.