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.