The other day I described a modern mainframe as something that resembles a cluster where CPUs are connected through very high-speed low-latency buses (to the point they can behave like a single-image NUMA machine or be partitioned into multiple smaller ones) connected to multiple networks of specialized computers each running their own tiny specialized OSs that manage IO over different buses. It's common for them to have hundreds of PCI-e buses (and cards connected to storage, communications, other mainframes - for failover or clustering - etc).
An x86 server is a computer built around its CPU. A mainframe is a computer built around its buses.
"Single system image" is a real approach to clustering, if a bit of an experimental one. There used to be a variety of toolsets attempting to provide this functionality under Linux (e.g. OpenMosix, OpenSSI) but they all seem to have bitrotted by now.
An x86 server is a computer built around its CPU. A mainframe is a computer built around its buses.