The Grub bootloader will have been installed to the EFI partition and registered as the default bootloader with your Mac's firmware as documented in the Debian Wiki here.
That's why your Mac is booting directly to elementaryOS.
To remedy this, you can do the next section of the tutorial, Install rEFInd Boot Manager, directly from elementaryOS.
Installing rEFInd should allow you to boot into macOS again.
Installing rEFInd on Linux is documented on rEFInd's website here.
Below I outline the steps you need to follow to install rEFInd on elementaryOS and boot back into macOS.
Boot into elementaryOS.
Log in and open Terminal.
software-properties-common package so that you can easily add rEFInd's PPA repository.
sudo apt -y install software-properties-common
- Add the rEFInd PPA to your system.
sudo apt-add-repository -uy ppa:rodsmith/refind
- Install rEFInd.
sudo apt -y install refind
Confirm when asked if you want to install rEFInd to your ESP.
That's all that is required to install rEFInd, so reboot.
When the computer is booting, it should show the rEFInd menu.
Select the macOS option and press enter.