"Friendly" in the HN title is inaccurate in my opinion. The paper asked participants to rate a person's perceived cooperativeness, not friendliness. While it's true that cooperation is a component of being friendly, the two are not identical.
For example, "he is cooperating with authorities" is very different than "he is friendly with the authorities."
With that usage, of course head-down position looks more subservient. A person who is fearful, or surrounded by a generally hostile population might perceive that as "more friendly" at a stretch of semantics, but that would say more about the culture and context.
My experience is quite the opposite. Heads-up with eye contact indicates an engaging personality, readiness to listen and see, and therefore a more sociable outlook. First thing you get taught in cadets or scouts or whatever is "heads-up!" - a ready/attentional posture.
Indeed, the widespread problem with smartphones is visible in any British high-street. People bimbling along, heads-down, necks craned, staring into the 6 inches of virtual reality through which they see the world. It's not only horrifying from the POV of their health and safety, it sends a "non-approachable" signal.
If I may follow the grandparent commentor and provide an unsubstantiated thought: the heads-up angle may suffer from being double-edged; it may indicate someone who suffers from excess self-esteem. From experience, these types are harder to deal with because many times one has to help them relieve themselves of their ego first, before anything substantive can be accomplished. Sometimes, especially as we age, the juice is simply not worth the squeeze and we’d rather just work with someone who is better ready to work together.
Sure, there's a definite line between heads-up and a cocky nose-in-the-air. As another commenter said, it's more than just head angle, you need to consider face expression and eye position too.
The expression "looking down at people" is probably captured in precisely that ratio of head-eye angle that gives off haughtiness, and the "chad" comic posted above sums up the excess of other body language that makes one send out "I'm a dick" signals.
In fact I heard that so many times on TV as kid in the 1970s and 1980s that I considered "capitulate" as a drop-in replacement for "cooperate". Only later did I revise the meaning as something that happens between parties on an equal power footing.
I guess that's what happens when people misuse words as a euphemistic way to soften intentions and behaviours - like "collateral damage" and so on, and then that culture gets exported through popular media.