Mildly Interesting

every line on graph 1 has a slope less than 1, so this is not a meaningful evaluation to determine anything, in and of itself.

It's meaningful to the only question I've asked, whether tall women prefer as large of an absolute height difference as short women do. The answer is no. Tall women prefer taller partners than short women prefer, but they prefer a smaller gap between themselves and their partners. According to the graph you posted (fig 1, which says it's the confidence intervals for "preferred partner height"). As the paper explains:

We found that male height was positively correlated (r = .69; p < .001; N = 188) and that female height was negatively correlated with preferred partner height difference (r = .49; p < .001; N = 461; ESM Table 2). Thus, taller men and shorter women preferred larger height differences, i.e. the male partner being much taller, whereas shorter men and taller women preferred smaller height differences, i.e. the male partner being only slightly taller (in line with Pawlowski (2003)).

So I think I'm reading that graph correctly and you're not. Your discussion of fig 2 seems to be talking about the part of the paper on people's satisfaction with their partner heights, which is a different metric than preferred partner height.

Everything else you're talking about is not particularly interesting to me, and wasn't what I was asking about.

Delta typically refers to change over time.

Delta just means difference. A change over time is the delta of that variable over delta t.