The Imperial Guard isn't necessarily a sensible military - depending on which world the regiment was raised on - hence its infantry does not need to conform to modern standards or contemporary wisdom. There are regiments that wear nothing but leather or fur, or go into battle bare-chested, or wear medieval chainmail. If you go by the current Codex, there are even IG regiments out there who go into battle with spears and bows, because that's what they use on their homeworld and the Munitorum didn't manage to issue the standard lasgun in time.
That aside, armour isn't everything. In WW1, German shock troops were issued heavy breastplates to protect against projectile weapons. In the field, the soldiers ditched them because they were limiting their mobility in storming trenches too much compared to the small protection they offered. I for one have no problem assuming that Catachans have a similar mindset - you don't need armour if you're behind cover (see hit & run tactics), and "realistically", that is in a world without P&P mechanics where people can be shot a dozen times without being injured, the vast majority of weapons in 40k do not care for a bit of flak anyways. There's a reason players call it "cardboard armour", as much as an improvement it may be above modern kevlar.
Besides, if Catachan Jungle Fighters are not employed in a jungle environment, well .. sucks to be them! It has always been a hallmark of the Imperial Guard that its many regiments all have different perks and perform with different efficiency depending on the environment they are deployed in. I wouldn't throw a unit of Rough Riders into urban combat and expect them to be fine either. But hey, it's war.
I would agree that "flak silk" is pushing it a bit too far, but that's a matter of personal taste. The fluff in 40k is as diverse as we want it to be and, by company policy, has room for anyone's personal preferences and interpretation. There isn't even such a thing as "outdated" fluff, simply because it's up to each of us individually to determine what we're going by. The only thing I'll add to this, however, is that I don't really see a point in re-writing the Catachans to better suit one's own ideas when you may just as well make up a completely new regiment as a fresh start. It's more creative and thus more fun, plus you manage to circumvent the issue of players being used to a different type of Catachans.