You may want to check out the video / text from everyday astronaut pointed to elsewhere in this post, which goes through a much deeper research and explanation

You may want to check out the video / text from everyday astronaut pointed to elsewhere in this post, which goes through a much deeper research and explanation on the topic. The TLDR as I remember it, is physical nozzles on the aerospike don't work the same way as they do on a traditional rocket. It's more like like just an exit from the combustion chamber. The appeal of an aerospike is by having lots of exhaust port aligned around a spike, is it creates a sort of virtual nozzle that is the right size for the atmospheric pressure. And as such acts sort of like a nozzle that changes shape and size as the rocket goes through the thinner and thinner atmosphere. This in theory gains efficiency over a rocket that has a set shape for it's nozzle, and isn't always operating at optimal efficiency due to the size of the nozzle. * Not an expert on these things, just going off of memory