kindles have displayed the book cover on the lock screen for approximately the last decade.
2021.
People have certainly been asking for it for at least the last decade. Jail breaking doesn't count.
It's not a new thing that journalists cover old features as if they've just been added, because they just found out about it and feel they're newsworthy
It's likely you are just lucky to buy a KPW with that feature just rolled out.
I only added my experience because bongobingo said 2021 and cited a tabloid "article" as proof.
The feature definitely predates April 2021, how long? I have no idea, possibly only 6-7 months, as I bought mine in October 2020.
More "tabloids" in April 2021 https://www.tomsguide.com/news/amazon-kindle-update-adds-kil...
More nerds in August 2021, https://forums.whirlpool.net.au/archive/30x1qmxj "the setting that was added a few months ago in Settings/Device Options to show the "Display Cover" of the book you are currently reading"
Amazon discussing rolling it out during ... April 2021, https://www.amazonforum.com/s/question/0D54P00008A1VpoSAF/di...
Maybe you were in a pre-release A/B or region, or simply didn't notice the update.
By all accounts your Oct. 2020 date is the earliest reported date for it being an official feature. Kobo has had it since day one in 2010, Kindle not so much.