This does nothing to shield Linux from responsibility for infringing code. This is essentially like a retail store saying the supplier is responsible for eliminating all traces of THC from their hemp when they know that isn’t a reasonable request to make. It’s a foreseeable consequence. You don’t get to grant yourself immunity from liability like this.
Shield from what exactly? The Linux kernel is not a legal entity. It's a collection of contributions from various contributors. There is the Linux Foundation but they do not own Linux. If Linux were to contain 3rd party copyrighted code the legal entity at risk of being sued would be... Linux users, which given how widely deployed Linux is is basically everyone on Earth, and all large companies. Linux development is…
However, there is no legal precedent that says that because contributors sign a DCO and retain copyright, the Linux Foundation is not liable. The entire concept is unproven.
Large company legal departments aren’t a shield against this kind of thing. Patent trolls routinely go after huge companies and smaller companies routinely sue much larger ones over copyright infringement.