The top voted comment from basically the same thread in 2017 [1] does a good job of presenting a point of contention in a relatively neutral fashion:

from the /u/aleyan:

> The figure was given in a secret diplomatic cable from then British ambassador to China, Alan Donald. > Mr Donald's telegram is from 5 June, and he says his source was someone who "was passing on information given him by a close friend who is currently a member of the State Council".

Here we have a telegram by a guy (British Ambassador) who heard from a guy (unknown) who heard from a guy (unknown State Council) facts about the events of the day prior (massacre was on June 4th). Where did the unknown State Council official get his estimates from; were those official or just something he heard and repeated (and when did he get them)? Initial estimates of disasters are often quite wrong; here they were produced in game of telephone in a day or less; and they are not collaborated by any evidence we have now.

I rank the quality of new evidence as low. Rumors repeated in old official telegrams are still rumors. I expected BBC to have reported more critically. Alan Donald is still alive; BBC could have asked him if he received any updates to that first number that he trusted more.

I also have to fault BBC for it's phrasing around Donald's source. At first reading it sounded like Donald's source is an unnamed member of the State Council who is a close friend of the Ambassador. After reading BBC's sentence a carefully however; it sounds like the Donald's source is a person who is a friend of an unnamed member of the State Council. This ambiguous sentence is deceptive.

EDIT: I see vote count moving up and down on this comment making me think it is controversial. If you disagree with my doubts on the veracity of this story, write a comment. Maybe I missed something.

[1] https://news.ycombinator.com/item?id=15993956