What you're seeing here is a contrast between spoken English and written English. In the written form, the writer can typically consider the thought in its entirety, and apply a grammatical rule to the entire construction. Doubly so if the work is proofread and corrected.
In spoken English, the thoughts are assembled as they take shape in the speaker's mind. Rather than defend myself is an exceptionally strong statement related to self-preservation (even an argument can count here), so as an idiom it takes a simple form that emphasizes the verb.
Also, in spoken English the speaker may not be conscious of what they will say next, so after a strong introductory phrase, it's possible to hear a wide variety of constructions, e.g.
Rather than fight, they chose to surrender.
Rather than surrender, they died to the last man.
Anyway, rather than die in silence, the sergeant called them to attention and began the first words of God Save the Queen. Didn't make any sense, really. Whole damn war was like that. But they sang it all the same.
Edited to add:
One could imagine a difference between the simple verb and the gerund as follows:
Rather than surrender, they fell upon their swords and died. Decision.
Rather than surrendering, they fell back gradually into the marshland, scattering onto paths where the mounted knights could not pursue them. Continuing action.
After writing these examples, it seems to me that the simple verb is used to express will of some kind. It carries more emotional weight, and seems more natural in spoken English.