I don't know about the timeline of Vimscript in particular, but seemingly Vim was first released in 1991 while Lua first 1993, so by the first release of Lua, Vim already existed by then. Not sure when Vimscript appeared though.
On other trends (Google Trends to be specific), search volume for Lua only became larger than for Vim in 2004, with Lua hitting it's peak in 2009. Search volume for Vim would have been much larger before 2004 if we had a single search engine everyone could have used, but hard to know in afterhand I guess.
vim9script was most definitely written way after lua was integrated into neovim, and well well after lua was popular as an embedded programming language choice. While it is plausible that the idea for vim9script was conceived without him knowing Lua was a choice, it is unlikely that work on it was started without Bram knowing. There's even people that have commented on the google groups as far back as 2016 about neovim's use of lua. It's unlikely that Bram wouldn't have seen that.
vim9script, though, is the evolution of vimscript, and has some amount of backwards compatibility, short-ish migration guides, etc. Initially choosing an embedded script language and migrating to a new one are fairly different scenarios.
Vim already could be compiled with Lua, Ruby, Python with their own respective vim APIs. How is it possible that he didn't know he could use an established programming language for scripting?
