Is MIMO equivalent to link aggregation?
MIMO is not the same as link aggregation.

How does MIMO work?

Imagine you have two radio channels that are physically separated, a Tx/Rx antenna pair here, and another pair 'over there'. The two channels will be essentially independent, even if they use the same frequency, and you'll be able to transmit a total of 2x the information that you could on a single channel.

Now bring those antenna pairs closer together. You can deal with interference between them by matrixing the signals, so that receiver A might get 90% of Tx A's signal and 10% of Tx B's. As long as you know the channel response, you can put the correct coefficients into the matrix.

Now co-site the antennae, but make sure that they are separated by at least 0.5 wavelengths. This means that if the path between Tx and Rx is sufficiently full of scatterers (cars, filing cabinets, people, window frames), their fading will be independent. Conceptually this means that each antenna 'sees' a different RF channel to the others. Once this complicated channel has been 'sounded' by a test signal, and assuming the that the channel characteristics stay steady for at least enough time to transmit a few frames, the signals can be matrixed onto both channels by the transmitter, and dematrixed by the receiver. You don't get 2x speedup with 2 antennae, 50% would be reasonable going, and it depends on having a sufficiently 'busy' RF environment between transmitter and receiver, line of sight does not work at all well for MIMO.