Is spatial multiplexing always required to define MIMO?
This a daffodils versus yellow flowers problem, all daffodils are yellow flowers, but not all yellow flowers are daffodils.
MIMO means 'Multiple Input, Multiple Output', no more than that.
Having many antennae can allow you to do various things, assuming you have the additional hardware and software to enable them

Spatial Diversity
Beamforming
Antenna Diversity
Frequency and Polarisation Diversity

In the particular case of your fixed network vendor, having FP diversity with 4 channels is a robust way to get a four times speedup on a fixed link, which is completely impractical on a link to a mobile. However, 4x4 SDM MIMO can at best get three times speedup and often less, but also works with mobile links. They are used for very different things, in different contexts.
Antenna Diversity has always been possible without 'clever' software, but for that reason it's not often described as MIMO.