That's one interpretation. Really, it's a justification of the weapon proficiencies.
The other is that the wizard had to work to learn their magic while the sorcerer is born with it. The sorcerer doesn't need to learn how to fight as they have the inborn power to blast their foes with magic.
Just to add to the above,
1)In many stories that D&D players might use for inspiration, inborn powered casters are often not trained in weapons and have to learn to control and develop their powers over time (either through experience or formal study); and
2) As for the spear, it makes sense for a sorcerer not because they are a gish, but because it would be a fairly common weapon among rural commoners so they would have some basic familiarity with it whereas the bookish wizard would be locked away in his or her studies with their masters and have an even smaller selection of weapon familiarity.
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