This whole story is silly. DOJ memos are not law, and saying Mueller cannot do something if he follows the DOJ memos does not mean he cannot do something. I'm not a supporter of indictment or impeachment based on what I have seen, but this story does not mean he will definitely not be indicted even if RG was telling the whole truth.
It's silly, but not quite for that reason. Although DOJ memos are not law, they do have a legal impact, in that they control what employees of the DOJ may and may not do (this was most recently clarified in the courts by Judge Alsup of the 9th Circuit in the DACA case), and that if they step outside those bounds their actions may not be legal.
Here's a link to the various memos in case anyone is interested. I wouldn't suggest reading them all - these are long-ass memos running to 255 pages in aggregate - if you're going to read one make it the Moss memo from October 2000.
No, the reason it is silly is that there's no complicated political process to go through to change the policy. You don't need a 2/3 majority or anything, you just
write another memo. Indeed the oldest one, the Bork memo, is from October 1973, that's 15 months
after the Watergate Grand Jury was impaneled, and Bork's argument was quite tied to the facts of the Nixon case. If, in the specific circumstances of the current investigation, the DOJ wanted to change its mind there's a template already there (the Rotunda memo to Ken Starr of May 1998, included in the above link) all ready to copy and paste.
I rather suspect that they would prefer to go the impeachment route rather than rock the boat, but they could do so - and that possibility might provide some pressure on Congress in impeachment proceedings.
Giuliani claiming there's some real immunity is just smoke, his claiming that Mueller said so is mirrors.