If you are as well versed in loshon hora as you claim, then you should be familiar with the safeguard of protective fences as being very liberal and covering a lot of ground.
Is it within the parameters of shmiras haloshon to want to be careful about speaking about someone who is anonymous to you? Absolutely!
Remember, the Chofetz Chaim also talks about concern for your “fellow” and does not stipulate your “fellow” means only if you know their name.
If someone wants to be careful…extra careful..that is meritous… Dont call them on the carpet for that or lose your patience.