It's not baseless, and I know you can do well self-publishing. That is precisely why I said you should sale from pages like Amazon. When someone makes a purchase on Amazon, it's unlikely that they will see the book is self-published.
If I'm talking to someone and they say "I just published my first novel," I'll congratulate them. When they tell me they self-published, my first thought is that they weren't talented enough to get a real publisher. This is how many people think. I'm not saying they're right or wrong to think that, but it's true. You can Google the issue and find multiple forums, blogs, etc, where some people will argue against self-publishing.
There is not a significant amount of people who will think self-published works are more likely to be good. This means that on average, a self-published work will be fighting harder for credibility. Hence, it's desirable to avoid giving people cues that the work is self-published.
Self-publishing can be successful, look professional, and produce quality content. And if you're selling something, taking out the middle man can be beneficially from a financial perspective. It's usually the opposite because people can't market themselves, but that's less of an issue for IMers. All I'm saying is you're making a bad choice if you tell your customers you self-published if it's avoidable. Nobody cares much about the publisher unless it's a famous one so don't mention it. Or use the phrase "published." Self-published by Gregory Smith shouldn't be at the bottom of your sales page.
Personally, I don't assume self-published books are of poor quality. I assume every book is terrible because the majority of them are. I'd consider self-publishing something if I took the initiative to write it. It's just that within the amateur literary community (the blog of some low-tier writer trying to make it big) has a significant percentage of people who don't look at self-publishing fondly. And these are the people who have those 10,000 amateur blogs that promote new books.
Don't let me scare you out of self-publishing. A good book that's marketed well will most likely earn you money. I'm just saying that you should self-publish in a manner that makes it less obvious you're self-publishing. In fact, talking about your publisher at all takes away focus from the work, arguably.
I do not mean "credibility" in terms of business relations. I mean people will pre-judge you. Gonzo makes good points about how publishing is a business. But I'm simply saying that a lot of writer's think being accepted by a publisher is some sort of indicator that the quality is more likely to be good. It's still somewhat true that the average self-published book is probably of lesser quality. I mean, Penguin Books doesn't pick up the grammatically incorrect ramblings of a preteen girl telling a story about how life is tough. They at least edit her novel properly before publishing it.