throwaway55780
In some of those corps there actually is a "dont ask dont tell" kind of policy around personal projects/blog posts/oss contributions/etc. as the company forbids them unless you have special approval. However on the flip side managers know that the people with those projects and experiences often have a stronger drive than those simply treating coding as a 9-5 job.
I also know of cases where the interviewers were very impressed by the candidate's Github repo/oss contributions but stated that the candidate would have to stop working on that stuff after being hired. I know people who upon graduation seperated their projects for this purpose, continuing some under a screen name and having other less important ones for resume purpose knowing that they may not be able to continue working on them under that name in the future.