You explain the issue yourself.

When we speak about humidity we usually mean relative humidity. Relative humidity is the ratio of water parts in a given parcel of air. As you try to say, higher temperatures can carry more water parts (not more relative humidty ). This concludes, that the same amount of water parts in a parcel of air decreases the relative humidity at higher temperatures. The relative humidity however is one significant comfort factor in the air. Each percentage of lower relative humidity makes flying more stressful.

I personally prefer wearing a sweater and taking a blanket and feel more comfortable in higher relative humidity rather than sweating in a shirt and a bermuda in lower relative humitidy.