Usually seed stage startup founders assume the roles CEO / CTO from the onset.

CEO is also an executive level position, but one that is expected to be filled in even a 3 person company. If his day to day work is technical, but he still has comparative decision making power, it is appropriate to denote that by naming himself CTO.

I agree with you. It sounds pretentious. Chief of what? Yourself? Chief by definition means chief of others also in a decision making position.

Founder is enough; Founder is a big enough title to encompass the challenge, struggle and reward doing something new, independent or fresh.