I wrote: "Some people think that Horatio WERE is 1 an alcoholic, but because he is so resistant to the effects of alcohol, he rarely gets drunk. ..."
Gymlish correction says that is false " 'IS' is the proper conjugation of the verb 'to be' in the present tense third person singular form. This is the best choice here"

I don't get it. Horatio is NOT an alcoholic, and the speaker does know that. So why not "WERE" (or WAS)?