This is a 1 taco deal until the developer responds to this or enables the regular login functionality for the appsumo offer.

Why is one forced to sign in with Google account when buying the Appsumo deal but when using the regular login or signup on their website one can simply do it with email?

It all looks either suspicious to me or a very strange mistake on the developer's side.

I also took a look at their policy and the language is weird in general. Very unclear:

"Further information about the processing of Personal Data
Selling goods and services online
The Personal Data collected are used to provide the User with services or to sell goods, including payment and possible delivery."