In the UK you can. If I said on my web site "MR is paid by Canon to push their product" then I would be open to legal action as that statement is libellous, since it is untrue (or at least I have absolutely no reason to suppose that it is true).
I think Ken needs to be careful what he says. Now that would be a first.
He gave you his reasons and evidence that from which he derived his conclusion. It's purely circumstantial, for sure. You would need to first prove definitively that his argument is knowingly untrue.
On the other hand, we can parse his position by putting it in the best argument form we can given the information, and then test it for validity and soundness:
His argument goes something like this, which is invalid:
(1) LL image quality test comparing digital to scanned negatives is wrong.
(2) It's easy to see that MF scanned negatives are much better in quality than the best digital file.
(3) Since it's easy to see this difference for anyone who understands photographic and digital principles that this is true, LL must be giving out bad information knowingly.
(4) One very good reason bad information like this would be given out by people who know better is that companies pay people to do it.
(5) Conclusion: LL is being paid by camera companies.
What I would do to refute this argument would be to attack the premises first. Is the information KR gives accurate and accepted? If not, then the entire argument fails and you need go no further with it. If so, then we need to look at the argument's form to make sure that even though the premises are true, the argument's conclusions follow a valid form, that is, don't make leaps.
So second, I would test the validity of the argument's form, since an argument can be valid and unsound, meaning the premise(s) are false, but the logic is sound. (e.g., Socrates was a man. All men are deers. Ergo, Socrates was a deer. False premise, with valid argument form. Everything follows with no leaps. In this case, the first premise's "predicate" is 'distributed" in the second and the second in the third (predicate logic). You can see the validity by exchanging "deer" with "mortal" or "All As, are B's. All Bs are Cs. Therefore, all As are Cs.)
We see here that KR's argument is unsound because his premises do not lead directly to his conclusion--that LL is paid off by camera companies does not follow directly from his premises (If LL is giving out bad information, there may be other reasons for giving bad information rather than being paid off).
So in summary, even if his information is correct, the argument does not follow to it's conclusion. But if his information is correct, it does call to question many things.