The Nvidia driver being installed (version 390.116) does not correspond to the currently loaded Nvidia kernel module (version 418.74).
That message indicates that you did install a new driver; And it also seems to be a downgrade from 418.74 to 390.116.
The guide you were following has a big bold warning before the instructions to installfor installing the 32-bit OpenGL libs:
Installing the 32-bit OpenGL libraries may uninstall the 64-bit drivers you may have previously installed. Installing the AMD graphics or NVIDIA graphics drivers for your system is recommended. The appropriate OpenGL library will be installed along with those drivers.
To rephrase that:
Install the 64-bit Nvidia drivers. You only need to run this next command if you are looking to install them manually.
So, it seems like you are now running Nvidia's 32-bit driver version 390.116 after running apt install libgl1-nvidia-glx:i386. (Because libgl1-nvidia-glx:i386 depends on 32-bit driver packages.)
You can just reinstall the latest 64-bit driver, since that will also provide the necessary 32-bit OpenGL libraries that Steam requires.