If my memory serves correctly [1] releasing the source proactively is not a requirement; the requirement is that the source be made available upon request. So presumably they wouldn't have to do this until someone demanded it.

[1] I'm not entirely sure which clause I picked this up from (it's been a few years), but this clause I can right now seems to say the same thing: Our General Public Licenses are designed to make sure that [...] you receive source code or can get it if you want it [...]