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.