They might not be in a position to relicense all of it, or might not be sure of exactly how to do so appropriately.

Large, living companies like Microsoft can work with their lawyers to confidently understand what they're releasing when opening up old code and indemnify theythemselves appropriately, but a troubled company on its last legs can't nexessarily budget for all that.

Commercial software of that vintage was not built from dependencies that were all open source themselves, nor were there necessarily contributor/contractor agreements that kept copyright in a suitable place for open source relicencing. They might have been prepared for explicit rights transfer to another party, and maybe disclosure as they're suggesting here, but relicensing is a different thing.

Very weird. They provide a very specific future date for this, and avoided using the term "open source". I can't recall any other company doing this. Most of the time, companies provide a github repository at the same time they make such announcements. Even for twitter, Elon Musk promised the algorithm would be opened, and then some time later it was just there. While generally it is a positive thing to see code being available, I wouldn't think too much into it until I can see the license and the code.