It pretty much implies that you're attempting to overtake existing competitors who have found product/market fit, where people have options to fill their need but do so grudgingly because of their UX quality. Doing that can be a solid business plan, but it's very, very different from the type of new-product-discovery startups that Reif Hoffman was talking about.
If the people are paying for an existing crappy product with poor UI/UX, then that illustrates a valid need and building an easier-to-use product can greatly increase the size of that market.
However, if currently people are not eager to pay for that need, then I'd wager that simply providing UI/UX/ease of use won't change that. If their need isn't sufficient to put up with the minor bother of sucky UX (at least for 10% of most needy part of the market), then that need isn't also sufficient to put up with the much larger bother of paying you adequately.