Like I say, the immunity rule is universally accepted in the political press - they've consistently reported on it for a weeks now. This is just one example from numerous that come up by Googling "Truss immune": https://www.politico.eu/article/liz-truss-prime-minister-uk-conservative-party-finished/ which includes the line "One mechanism under discussion is changing party rules to allow for Truss to be challenged — ordinarily she is immune for the first year of her premiership — and for Tory MPs to choose her successor without a vote by the grassroots membership."The rules are there - published in the HoC library. I have linked them twice. I have read the rules. There is no immunity within the published rules for electing a Tory leader. If someone has decided to "invent" additional rules to suit the situation that is a different matter entirely.
If they played by the published rules, you wouldn't have to rely on rumours!
However, looking like it might be a moot point: reports that Brady has just gone into #10.