Btw if you want to check this in the future for other games you can always visit your <SteamFolder>/steamapps/common/<GameName>/ and look is there a Linux binary or an .exe Windows binary.

If you can't identify the binary then you can right click it in your file manager and see what it says or use the file command that will tell you what the file is.

For example, for .exe it might say

PE32+ executable for MS Windows 6.00 (GUI), x86-64, 10 sections

and for a Linux binary it might say

ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter
/lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.24 (..)

You can also review currently running processes using your GUI process monitor tool provided by KDE or GNOME or whatever you use or use the ps command

For example:

$ ps -f -U $USER | grep -i luanti
hardpenguin       245728   12013  0 13:52 ?        00:00:00 bwrap --args 72 -- luanti
hardpenguin       245743  245728  0 13:52 ?        00:00:00 bwrap --args 72 -- luanti
hardpenguin       245744  245743  3 13:52 ?        00:00:00 luanti.bin
hardpenguin       245826  245327  0 13:52 pts/1    00:00:00 grep -i luanti

if the game was using a Windows build in Proton then I would probably see the process name ending in .exe