It is just a simple topic starter. Not "how to fix the problem", but instead "why should anyone think it's a problem in the first place".
I agree. This is a valid strategy for winning, that doesn't always work 100% I might add. I will use it depending on various game situations, like who else is putting cards down, how many cards they have in their hand, what point value are the objects that I have completed and displayed versus completed and am holding...
BTW, a relatively easy way to determine if "hoarding" is going on is to check the status of their bamboo. If they have a bunch that has not been turned in and they're holding panda cards, guess what? They've probably completed one or more of them. Hording land cards can either be an indication that they're hoarding completed cards, or are holding cards they can't (or won't) finish. How to tell? Are they still pulling land hexes and playing them? Are they trying to irrigate? They're probably trying to complete an objective. Neither of the above? They're either holding completed cards or have given up.
It's difficult to hoard gardener cards simply because the state of the board can change, so you can generally rule that out.
Bottom line: it's not really a secret if you consider the indicators. Plus you can't hoard everything because of the card limit.
If you see someone hoarding, play accordingly. Easy enough.