Which is why no District Attorney or prosecutor would touch it with a 10 foot pole. They would end up spending tens of thousands of dollars or more, on what amounts to a frivolous, unwinnable case. Not to mention it would go totally against public opinion. Especially today, with all of the nonsense going on in the world that actually needs legal and financial action.
Well, I don't fully disagree with you that most prosecutors would see that for the can of worms that it is. But... that's poor advice to broadcast wholesale nationwide. In general, the feds far under-prosecute lies on the 4473 in the first place. And I don't mean for petty stuff like confusing CBD users, I mean for people who indicate they are not under indictment when they are (among other things).
The advent of the CBD industry has thrown a grenade into a lot of the laws on the books, both at the state and federal level. I am of the opinion that, for the sake of clarity, marijuana will have to be de-scheduled eventually. The .3% or less THC products that are now legal in most jurisdictions look, smell, and feel identical to stoner dope. I live in a state that has exempted CBD products so long as they contain less than .3% CBD. Since allowing that, a number of herbal wellness businesses have sprung up. Many of them are looking to legitimately expand into this newly legal market and comply with all laws (legal compliance in this arena is somewhat of a bear right now in my state). Many others are looking for a nice legal looking front to run a glorified head shop, banking on the plausible deniability of not "intentionally" selling illicit drugs. The difference between the two is not always so obvious.
Products include hemp flower, most of which is compliant with the .3% or less rule. Some of it is not, however. It looks, smells, and feels just like marijuana bud. Most shops give their customer a little slip of paper noting that it is a legally compliant CBD product to present to the police if it is ever found. As you can imagine, this doesn't work with all police. As you can also imagine, the packaging and legal compliance pamphlet often accompanies marijuana with THC to try and fool police.
On top of all of this, many well meaning shop owners who strive to comply with the legalities must be very careful and strict as to where they source their product from. Many products advertise as being CBD only, when in fact they have THC levels WAY over the limit. Something tells me there is some underground knowledge as to which brands misrepresent, and these brands misrepresent purposefully. Some shop owners who are less into the "high seeking" culture may purchase wholesale amounts of this illicit product believing it to be in compliance with the law, when it is not. If they are ever investigated or the product is tested, the excuse "I didn't know, and it was advertised as legal" may not prevent them being criminally charged. Even though they may not be convicted, they will spend time, money, and hassle going through the legal system.
This has a plethora of affects on many laws, including the prohibition on possession of firearms while being an illegal drug user. Drug tests typically do not get reported to NICS or any other LEO agency, unless you work for one or are on probation. If it's the latter, you likely can't possess a firearm legally anyway. At any rate, I am of the opinion that the marijuana plant needs to be either entirely legal or entirely illegal for the sake of clarity. This halfway stuff makes legalities surrounding it a mess. Which is why I believe the OPs question can't be answered for 100% positive. I don't think a prosecutor would ride to the gates of hell to prosecute you for possessing a firearm while using CBD products with .4% THC instead of .3%. But I can't promise that. This is legal issues that cannot be predicted with certainty across all jurisdictions. You also cannot count on lack of public support in every jurisdiction. Older active churchgoers in rural Southern areas will likely see anything that looks like marijuana as still being the "devil's lettuce," so if they wind up on the jury all bets of instantly winning is off.
All of this, but I still think there is like a .01% chance of getting in legal trouble coupling CBD products with firearms. But I think there is a chance.