Err, no, this is absolutely NOT true. "Fair use" does not mean "I think it's fair to do this", it's a specific exception to copyright rules. And it does not include things like "I couldn't buy
GW's limited-edition product so I have to use their IP". The actual situation:
Making your own cards with functionally identical but not copy/pasted rules: 100% legal. Game rules can't be copyrighted, so a card with "objective #4" on it is perfectly fine. Copyright would only apply if you used the same rules text as
GW, since even though the rule itself isn't copyrighted the text
GW used to describe it is. Also, note that this would include selling your cards, putting them up for people to download, etc.
Making your own cards with
GW's rule text for personal use: who cares if it's legal, nobody is even going to consider taking legal action against you.
GW's legal department might be a bunch of abusive
TFGs, but they aren't quite stupid enough to waste money on something so trivial.
Making your own cards with
GW's rule text for sale/distribution or paying someone to make your cards with
GW's rule text: illegal, and
GW's lawyers will have an easy case against you if they find out and care enough to take action against you.