1) This is almost true. Anything that is committed to Git is recoverable. When you "re-write" history, Git is creating a new set of commits in the history, an "alternate history path." It does not destroy the original commits, but there is no named reference to them (unless you created a branch/tag pointing to this line of commits).

2) In this case, if you want to actually destroy these unreferenced commits, you must run "git gc". This IS a destructive command. It will remove any unreferenced commits from the repository. (gc = garbage collect). If you never garbage collect, you will always have access to anything that was ever committed. It just might be hard to find since the only reference is the ref-log (if it was recent) or the commit hash.