The scarcity of college graduates with FOSS experience

The distinction between SE and CS is much blurrier in practice, and in many cases there is not an option between either - only one is available, so content from both is mixed, proportion varying wildly depending on location and course background.

Either way, FOSS can still be quite valuable even looking strictly at CS. It includes many areas of research that do involve constructing software, even if purely for exploration, evaluation of practical applicability, or a subject onto itself. Knowing and using tools that are freely available means the results are much more accessible and useful for other researchers.