I erased an APFS Volume containing a second OSX installation (trying out Mojave) but my Mac keeps trying to boot into it
You can change the default to boot High Sierra by either of the following:


Select High Sierra from the Startup Disk pain of the System Preferences application.
Hold down the control key when selecting High Sierra from the Startup Manager.


You still have Mojave code in your APFS container. In your case, do the opposite of the answer you linked to.


  BTW: You can also elect to not delete this code. The code will not interfere with the operation of the Mac and occupies very little space.


IN OTHER WORDS:


  Note: Below I have assumed APFS Volume "Preboot" is disk2s2 and APFS Volume "My Recovery" is disk2s3.


You can use Disk Utility under High Sierra to remove the APFS volume containing Mojave. However, removing Mojave also requires steps where commands are issued in a Terminal application window. Below are the remain steps needed to remove Mojave.


Open the Disk Utility application.
Get the file system UUID of the High Sierra volume . Right click the icon for the High Sierra volume in the Disk Utility application and select Get Info. Here, I will assume the file system UUID is 2E0E6320-5D0A-4181-B465-ABFF2CBBFC27.
Boot to macOS Recovery.
Open a Terminal application window.
Enter the command below to disable System Integrity Protect (SIP) on next restart.

sudo  csrutil  disable

Boot back to High Sierra.
Open a Terminal application Window.
Determine the UUID of the deleted APFS Mojave volume. Start by entering the commands given below.

diskutil  mount  disk2s2
cd  /Volumes/Preboot
ls


The UUID of the deleted APFS Mojave volume should be the same as the name of the directory (folder) that is not the UUID for the High Sierra volume (2E0E6320-5D0A-4181-B465-ABFF2CBBFC27). Here, I will assume this is 832D1AE3-C9F2-454F-BE45-64E1ECBA38AA.
Enter the commands below to remove Mojave from APFS Volume Preboot.

rm  -Rf  832D1AE3-C9F2-454F-BE45-64E1ECBA38AA
cd  ~
diskutil  unmount  disk2s2

Enter the commands below to remove Mojave from APFS Volume My Recovery.

diskutil  mount  disk2s3
cd  "/Volumes/My Recovery"
rm  -Rf  832D1AE3-C9F2-454F-BE45-64E1ECBA38AA
cd  ~
diskutil  unmount  disk2s3

Reset the SIP settings, by entering the command given below.

sudo  csrutil  clear

Restart the Mac