It is extremely optimistic, to put it mildly. You can definitely get inter-LPAR noise. I've seen it first hand, personally, on systems that I've worked on.

Moreover the norm for zLinux system is to isolate guests with zVM, not LPARs. LPARs are more likely to be boundaries for chunkier workload definitions - that advice may have changed since I last cracked open a redbook; zVM offers similar isolation to what you'd see with KVM, VMWare, or other hardware-assisted hypervisors.