Any time it comes to the keeping of animals, you will run into opinions. To some people, our saying that bettas ought to be kept in at least 2.5 gallons sounds ridiculous when they have kept bettas that were clearly happy in a little box for three years. The people on the PFK forum no doubt have their own reasons for believing what they do, and I am sure that, for the most part, they are very good reasons.
And for the record, I went to the forum and found the posts you are talking about, one person said that in their opinion the tank was too small to keep any fish in at all other than shrimp, and they were not rude about it.
I would not be overly concerned about a single person's opinion in this respect, especially when that person also says it can be done, and well. When you put a question out on a forum you are opening yourself up to comments from everyone, and you aren't always going to like them all, or agree with them all. That's fine, but it doesn't make a person rude for expressing their opinion when you have requested it.
In any case, I wouldn't worry about it. 7ish gallons is fine, in my opinon. My boy is in a filtered 6 gallon (6.5"x24")