Why swing-arm missile launchers could reload at sea

Could older swing-arm missile launchers be reloaded at sea even though VLS usually cannot?

Yes; their internal magazine and handling system could accept missiles transferred aboard.

Older swing-arm launchers drew missiles from an internal magazine equipped to handle and feed rounds. The ship therefore already possessed machinery for moving weapons between storage and the launcher. If replacement missiles could be transferred aboard and placed into that magazine, the complete system could be replenished at sea. Falklands-era reloads demonstrate the practical difference from vertical-launch cells that lack an equivalent onboard handling path.