I applaud the effort and entrepreneurial spirit and even the quality of the UI work. I'd question the judgement when there are so many to-do apps out there already.
I am agree with this... It's great staying strong in the things that you believe. I am not sure about questioning him though. Starting in a dense market is not an excuse for failure. There were too many "searching engines" back in the day and that wasn't a problem for Google.
Also, companies often evolve in different directions from where they start. Take Instagram for instance, they evolved from a social network and in the middle decided to take another path.
I don't think this is suppose to be an email killer. If they are doing this because of what PG said then I am afraid they might misunderstood him.