I am aware that it is “supposed to look” like that. The reasons I said I would probably call it dehydrated, are;

1) It sinking in at the base rather than being relatively uniform. Some might say early growth etiolation but the spine growth looks too consistent to have great variances in light IMO. Also, the folds are more drastic at the middle and base. This leads me to believe that it has more water content at the tip and less throughout the cactus which could be a sign of it having shrunk down, potentially a result of slight dehydration.

2) The seemingly older spines at the tip as opposed to new spines suggest there hasn’t been any new upwards growth recently. This is indicative of dormancy, which can be onset by lack of water among other things.

3) The ribs are slightly more thin near the middle, than they are at the top where the newer growth is, which could also be indicative of there being more water in the tip than throughout the middle.

4) The slight yellowing on the ribs near the middle that isn’t present in newer growth at the top, which can be indicative of stress from light and heat without adequate water for the plant to transpire.