Well, the thing about 1.0 (and 1.0's opening movie only reflected that) was that many things shown in it, never ended up actually being inplemented into the game itself. The large sword Derplander was shown strapped to his back in the prototype Drowning Wench (before his Echo trip to the Battle of Silvertear Skies) seems to reflect that. However,
numerous propaganda artwork for 1.0 (one such image was even also used for the US 1.0 game cover) shows Derplander clearly wielding a normal sword single handed, all but confirming his original class is GLA.
And for what it's worth, although the classes of his elezen and miqo'te pals in the Adventurer's Guild is unclear, in the Echo scene it's easy to see the classes of the party he joins who fight that malboro (the roe is a MRD, the elezen a GLA, and the lalafell a THM - based on the wand she has - and remember, this is the original 1.0 Armoury System being depicted here - any class could equip any ability freely, thus the odd casting of Protect by a
THM or a GLA using Esuna was actually justified. Incidentally Derplander's 'Enfire' arrow was actually a planned spell for 1.0 which alas, was never implemented in the game itself (although the scene was also meant to depict the long scrapped 'Battle Regimine' system that 1.0 had at the start).
Either way, he really started as a GLA (with a shortcut into ARC within his Echo trip), then went MRD upgrading to WAR, then LNC-DRG, PGL-MNK and then SAM and finally DRK.