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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.

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

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.

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