The minimum contrast ratio in relative luminance that the themes conform with always refers to any given combination of background and foreground colors. If we have some blue colored text next to a magenta one, both against a white background, we do not mean to imply that blue:magenta is 7:1 in terms of relative luminance. Rather, we state that blue:white and magenta:white each are 7:1 or higher.
The point of reference is always the background. Because colors have about the same minimum distance in luminance from their backdrop, they necessarily are fairly close to each other in this measure. A possible blue:magenta combination would naturally be around 1:1 in contrast of the sort here considered.
To differentiate between sequential colors, we rely on hueness by mapping contrasting hues to adjacent constructs, while avoiding exaggerations. A blue next to a magenta can be told apart regardless of their respective contrast ratio against their common background. Exceptions would be tiny characters in arguably not so realistic cases, such as two dots drawn side-by-side which for some reason would need to be colored differently. They would still be legible though, which is the primary objective of the Modus themes.