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 funded by large companies with big legal departments. It's safe to say that nobody is going to be picking this legal fight any time soon.
The Linux DCO system was designed to shield Linus and the Linux foundation from copyright and patent infringement liability, so they were certainly worried that it was a possibility.
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.