Well:
Read literally, they cannot "hold" weapons in their secondary arms (at least in combat), not that they "don't," and its because of a lack of strength, not a lack of dexterity or opposable digits.
As a RAI guy, I honestly hope they meant "wield"- better something than nothing, IMHO- but you know as well as I that the RAW crowd is out there...and that if the designers actually meant what they wrote, some people could be in for a serious surprise down the road.
Ah, here's the difference. You read:
"In combat, thri-kreen
must hold weapons and shields in their upper limbs..."
Whereas I read:
"In combat, thri-kreen
typically hold weapons and shields in their upper limbs..."
It's the second part of that sentence that feeds into both interpretations, but ultimately, I feel (along other evidence) vindicates my own. If they just stopped at "...since the middle pair lacks the strength for this purpose..." I might be more inclined to read it your way, but the last part of that is telling, in that it was even necessary to mention that "...using both sets of limbs would be awkward and unwieldy."
The implication isn't that they have this nearly-useless vestigial weak flabby arms hanging around that's only good for juggling weapons around. The implication is that it's just be too awkward to be swinging all four arms around like that given both sets' placement on the torso. Since the upper arms are bigger and stronger, it simply makes more sense for thri-kreen to rely on the upper set in combat and not let the lower set get in the way.
And again, since both the artwork and the
Multiple Arms feature both seem justify the fact that thri-kreen are fully capable of holding weapons in their lower arms, they just generally don't use those arms
in combat (note: the first two words of your quoted sentence.) Had the designers had the foresight to use the word "wield" in that sentence this wouldn't even be an argument, but I'm fully convinced that the designers don't realize that simply can't use established terminology in a conversational manner or in fluff text without players latching on to the very narrowly-defined literal meanings they've established for those words; for an example see the "what's an attack versus an attack power?" debacle stirred up over the recent magic missile "errata". I've learned from that event not to attach any particular meaning when a term like "attack" or "wield" or "hold" gets thrown around in any section that isn't specifically related to game mechanics.
Of course, even if the evidence doesn't strongly support my interpretation over your own (which I believe it does), there's still clearly enough there on both sides to support either interpretation. At that point, why not just stick with the one you like?