[blastofpast]

Ah, I believe I understand now. Here's an older tweet from Musk which mostly explains it: "Crush core is aluminum honeycomb for energy absorption in the telescoping actuator. Easy to replace (if Falcon makes it back to port)."

[greglindahl]

This was a hot enough landing that they used a 3-engine hover-slam to reduce fuel usage. Their last attempt at a 3-engine landing burn was also their last landing failure. When a landing goes slightly wrong, the crush core is the first thing to go... and this time it successfully did its job.