Your post links to a BBC News article.
https://www.bbc.com/news/world-54422023
The BBC News article was reporting about a special meeting of WHO leaders that took place on October 5-6, 2020. The BBC article was posted on October 6, 2020.
The WHO meeting and its goals are described here.
https://www.who.int/news-room/events/detail/2020/10/05/default-calendar/executive-board-special-session-on-the-covid19-response
The BBC News article stated that:
“Just over 35m people have been confirmed as being infected with
coronavirus - the WHO's estimate puts the true figure at closer to
800m.”
The BBC News article stated that:
“The estimate that 10% of the world's population has contracted the
virus came from Mike Ryan, the executive director of the WHO's Health
Emergencies Programme.”
The documents from the WHO meeting on October 5-6, 2020 can be reviewed here:
https://apps.who.int/gb/e/e_ebss5.html
See in particular the technical report.
There is nothing in the posted meeting documents that explains the 10% estimate that is attributed to Mike Ryan.
The WHO October 2020 Situation Report also does not contain a mention of the 10% estimate in the BBC News article that is attributed to Mike Ryan.
https://www.who.int/emergencies/diseases/novel-coronavirus-2019/situation-reports
As of October 9, 2020, there is no WHO News Release that mentions the 10% figure.
https://www.who.int/news-room/releases
You are correct. There is nothing identifiable at WHO that explains the 10% figure.