I think that at least part of the benefit from telling yourself "I don't get ego depletion" is from telling yourself "I don't accept ego depletion as an excuse to stop working".

If you model motivation as trying to get long-term goals accomplished while listening to a short-term excuse-to-stop-working generator, it matches up pretty well. I did a short test just now with telling myself "I don't accept being tired as an excuse to stop rasping out a hole", and I noticed at least two attempts to use just that excuse that I'd normally take.