either way, whatever leads to a dehydrated state does not help the packing between the vertebrae, why people get shorter as they age, the packing, disc things get thinner with obvious results, less shock absorbing, tie that with aged bones and we have breaks of less flexible areas which is common in the elderly, hips and knees leading to replacement surgery. My mother has just had two knee replacements done.
I suppose the rule of thumb is if you feel the need to drink, then you are dehydrated, but if you are constantly drinking then something is wrong let alone ones back disc things.
I think you're conflating things, that's all.
If I was to speculate, most people who have back issues, it tends to be soft tissue / muscle.
Now I can't say as I've ever researched the impact of dehydration on people who have back issues actually related to the spine itself, but what I would say is that for most people (unless to some weird excess) that drinking caffeinated beverages will have precious little effect on them being dehydrated, because the amount of liquid in them vastly compensates for the mild diuretic effect of their caffeine content.