Mmmm I think there's morsels of truth to the statement, but I think it's also very relative and on the subject of webcomics I don't think it's very accurate (and even on other topic areas it depends).
On paper I think it makes sense as like... an idealized economic principal. Theoretically if a good or service has an accepted standard rate, then most of the time people who price their product/service above that rate will have a hard time making sales, and those who under-price might see more sales because it's a good deal. But like the average should be the price that you're seeing most of the time. The problem comes when the quantity of people offering under-priced goods is so plentiful that the average consumer doesn't need to pay the average price, because there are always the under-priced options available. In that way the lower prices become the new average and anything above that will struggle. This can even trend far enough downwards that you see people overing goods and services for free just to try and establish themselves as noted in the OP, and that's overall bad for an industry because it crashes the perceived value. It takes a lot more work to try and raise that bar again if it's dropped that far than it does to lower it in the first place, as consumers will be reluctant to pay more for the same goods.
HOWEVER there are a few caveats. The first that's pretty widely applicable regardless of the subject matter is that there are just way too many individual people working in any established industry to feasibly corral all of them together to get on the same page about pricing. Even if a set average rate is decided upon by the key/major players, there will always be those that go out of their way to undercut a little bit to get an edge on the competition. Whether it be a lower average price, or periodic sales, or what have you. The above principal could possibly work in like... a closed and very controlled market, but in a free market it just won't. That's not how the human race works.
The other thing is that the principal strikes me as something that's mostly applicable to professional products and services, and not so much stuff that people make as a hobby or for fun. Like even if that were to be enforced in the professional realm, I don't think it would be reasonable to basically tell people "you either have to monetize that or you can't post it". If an artist or musician or writer or whoever wants to make things and share them for free, they can do so.
I think the place I've seen this talked about the most in the past (personally) has been on art forums when talking about art commission pricing? I've read word for word "don't under-price your work because it hurts the art community as a whole!" several times when looking into the subject. And to a degree it's true? Even then though there are several circumstantial factors that can play a part. 3 come to mind right away:
1.) It's not uncommon to see the radically lower prices being offered on products or services that are comparably lower quality than the "average". It's like you get what you pay for? People who need to try and shill their product or service but are still developing can't necessarily get the same rates and the veterans in their field, so they'll either get no sales at that price or need to lower it.
2.) Similarly, sometimes people slash prices out of desparation, if they need to make ends meet, or their store is going out of business and they need everything gone, or what have you. It's not ideal, but definitely understandable.
3.) Situations where you go down as low as "free" are also like... sometimes necessary evils for getting your foot in the door. Whether you're handing out free merch at a convention booth to get your product in people's hands, or posting a bunch of free illustrations to advertise your commission business, or working an unpaid internship to get your foot in the door at a company.
Alrighty so like looping back to webcomics... I think several of the above can apply, depending on the creator's situation. Maybe they're providing free content for fun or as a hobby, maybe it's because their skill level isn't high enough to garner payment on level with the pros, or maybe they're putting out high quality free work as a means to get their foot in the door? Regardless of the situation I don't feel like it's particularly detrimental to the industry at large. Especially one still as particularly niche as webcomics in general. (I feel like print comics are a different animal and this post is long enough so I'll call it good there lol)
