This works to different degrees with different nouns.
Both en öl and ölen work fine. We normally say en kaffe about a cup of coffee, but not kaffen about that same cup in the definite form. And for wine, we don't even say en vin.
What I said here is true for general usage. The jargon among wine afficionados, restaurant staff etc is another question, people who specialize in something will always express themselves in ways that differ from the average usage.
The first part has some truth in it: Many en nouns will have the -en suffix as definites. en bok, boken 'a book, the book'. But many others will end in -an. en klocka, klockan 'a watch, the watch'.
The last part is not true. Many ett nouns are countable. ett äpple, två äpplen 'an apple, two apples'.