[javagram]

Gravity pulls blood down, your circulatory system works to more evenly distribute the blood but is impaired when sitting.

The same phenomenon is observed when sitting on an airplane for a long time resulting in slightly enlarged feet due to blood distribution.

[ukulele]

So then wouldn't the mechanism be that less blood is able to flow upward due to pressure on the skin?

[VygmraMGVl]

Blood flows in veins due to contractions of surrounding muscle. When you're sitting, your lower body is largely relaxed, which causes blood to pool in your lower body. It's a serious (acute) issue in people who are bedridden for long periods of time. I suppose the article is suggesting there's a downside to chronic, low grade exposure as well.