It's not really about the claim being "true" or "untrue". It's about being clear from the outset, based on their obvious conflicts of interests, that this organization is only going to report on study outcomes that benefit their perspective, even if they are true. For example, given all the evidence I've seen on this topic, I believe all of the following are highly likely to be true:

1. Disposable plastic bag bans significantly reduce plastic bag litter and its effects on urban quality of life and the environment.

2. Most reusable plastics bags are only used once or few times before they are discarded.

3. Given #2, the amount of fossil fuels used to produce the reusable bags makes them a net negative in terms of greenhouse gas emissions.

The basic problem with all discourse these days is that depending on your "side", you only talk about the items that benefit your viewpoint. American Recyclable Plastic Bag Alliance only talks about #2 and #3. At least the group referenced in the original article agrees that the current situation leads to more plastic being generated and should be corrected:

“Grocery stores, restaurants and retail shops should not be permitted to distribute plastic film bags of any thickness at checkout. Stores should be required to charge a fee of at least 10 cents for single-use paper bags. A 10-cent paper bag fee will limit the expected increase in paper bag use after a bag ban is imposed and may even reduce paper bag consumption altogether.”

It doesn’t matter nearly as much as you think where they are coming from as long as yourself follows on logic and facts. Almost every one in such a discussion is biased. You can still point out what they ignored or misled than merely stating a universal argument when you disagree.

The point isn't that the original commenter doesn't like them, it's that this is a lobbying group. As such, they have no credibility on this topic. So, in any discussion involving them, everything they say needs to be looked at as a ploy in support of their agenda, because it is their job to do that. To treat their word the same as anyone else's on this topic would be very stupid.

Pointing out that research was paid for by an organization that has an inherent conflict of interest is an extremely valid argument. It doesn't necessarily mean that the research was biased or shoddy, but it absolutely should cause us to take their conclusions with a healthy helping of salt.

You will still need to point out the actual ignored facts or false claim to substantiate the bias claim, otherwise it is an extremely cheap argument that everyone else could use the same argument as most people represent some interest group.