Pure speculation but you can get and click a VPS/dedicated server with a 1Gbit/s pipe almost everywhere. Most of these machines are not regularly patched and run often vulnerable PHP software.
I guess if you have a few 0-days for common stuff that is often hosted you can easily!? collect a few thousand servers that blast out traffic.
Additionally more machines have faster access to the internet as fiber gets more popular, if you have a decent sized botnet that can pump out traffic on average with 50mbit or so you can also get some serious traffic, not sure about getting 620GBps, through. I guess if you mix all of these and put some amplification attacks on top it's possible.
I guess if you have a few 0-days for common stuff that is often hosted you can easily!? collect a few thousand servers that blast out traffic.
Additionally more machines have faster access to the internet as fiber gets more popular, if you have a decent sized botnet that can pump out traffic on average with 50mbit or so you can also get some serious traffic, not sure about getting 620GBps, through. I guess if you mix all of these and put some amplification attacks on top it's possible.