[Rusty]

If you do a CREA search you'll immediately learn that the not-fully-quoted reference material posted in the other forum is misleading. There are CERTAINLY times when those verbs will be accompanied by an object that is preceded by an indefinite article.
All of the examples listed above by Angélica are a good indicator that incorrect information was provided in the other forum. I had a look at the grammar book myself and saw 'blanket' statements that should have also been quoted. I also found that they listed exceptions to 'the rule' in the self-same section. Only when talking about common/expected things, in general, and on an individual (singular) basis, can the indefinite article sometimes be omitted.

Context matters a great deal. There are times when the indefinite article is omitted and there are times when it is not. You need to accept that context drives usage.

I've also noticed that Duolingo has canned answers/translations that they'll accept, and will flag some valid variations as incorrect. In fact, the forum seems to contain a lot of 'why was this flagged incorrect' questions. The best answer often turns out to be that Duolingo could improve, because they didn't take into consideration that context is key. Learning a word, phrase, or sentence out of context is simply not adequate!

The only case I can think of where you should NOT use an indefinite article is when you're talking about what someone does for a living or when communicating social status.
Soy médico. Soy profesora. ¿Es carpintero usted? Soy estudiante. Soy soltero.