The only plausible alternative would be to send out an email with a download link in it, but that "costs" the designer an email credit, real money, or potentially extra development time vs just shipping with an out of the box multi-auth component built into some web framework.
Perhaps I don't understand your goal: are you trying to collect email addresses to build a marketing list, or are you trying to give stuff away for free?
Sorry if these are dumb questions: the website doesn't really explain the business model (if there is one), or if this done out of the goodness of one's heart.
I imagine you want people to try a free template and then ask for your services to customize it? Is that how it works?
They could ask for my email address. That is not a problem. But I won't "log in" with my Google account and potentially leak, now or in the future, my contacts and other private information.