Miners aren’t supposed to agree on a source of time like NTP. The current FTL could be used in an attack if they did (for example to split the network hashrate).
I get 380 ms as a delay that’s between the median and mean of the propagation delay. I used the data from this paper to see the exponential CDF might serve as the estimate from the orphan rate. This page shows about 1 orphan per 1575 blocks in the past year.
orphan rate = 1/1575 =~ 1-e^(-delay/600)
delay = - 600 * ln(1 - 1/1575)
Detecting selfish mining: If there’s a > 33% miner doing a minor selfish mining attack by withholding his found blocks for 1 or 2 seconds, it artificially increases the orphan rate which increases the estimate of the delays. You would have to know the propagation delay from some other method to see if it agrees with the orphan rate to detect the presence of a selfish miner. My +/- 10 s rule wouldn’t help if they’re holding blocks only 1 or 2 seconds. Another rule to help stop an attacker who’s causing tip races is for miners to choose the tip who’s arrival time is the closest to the timestamp, and change tips if he discovers the majority hashrate chose the other tip.
The newsletter mentioned monotonic timestamps might cause a problem if a timestamp targets the FTL. That would require finding a block less than 1 second after the “FTL block” and would be ignored for less than 1 second so I’m not sure what they have in mind. Does a block beyond the FTL have a penalty box where miners don’t work on it for some period of time?