It's a text command driven game, isn't it? Getting an encyclopedic listing of all of the different text that the original recognized would be most easily handled by reading it from the original code.
I suppose just examining the original media might have been enough. Or maybe they abandoned that command style? I didn't play the remake.
In 1991, they remade the original Leisure Suit Larry to be a traditional point-and-click adventure game. It's referred to as Leisure Suit Larry 1: In the Land of the Lounge Lizards or just Leisure Suit Larry 1 VGA. The modern-day remake was an update of the point-and-click version.
If they wanted to referrer to the original dialogue or images, it's easy to do without the source. ScummVM and other projects already have tools out there to extract all the game data from old Sierra games.
I suppose just examining the original media might have been enough. Or maybe they abandoned that command style? I didn't play the remake.