Two different ways of putting multiple antennas to work — one makes a single link more reliable, the other multiplies how much data moves across it.
Say a system uses "multiple antennas" and you still haven't said much. Diversity reception and MIMO both put more than one antenna to work, and both exploit the same underlying phenomenon — multipath propagation — but they were built to solve genuinely different problems. One takes a single data stream and makes it dramatically less likely to drop out. The other takes the same multipath environment and uses it to carry more data at once. Assuming any multi-antenna system is automatically doing the second thing is one of the most common conflations in wireless engineering.