(I know the instructions used v3.3.5, but I just grabbed v3.3.6 since it’s the latest.)
After installing rbenv & ruby-build per the instructions (bottom of page 8, top of page 9), I restarted my Terminal. Just a note: I never type exit + Return (as suggested on the page) to quit the terminal. I always use Command+Q, and just click the icon in the dock to relaunch.
I then installed Ruby 3.3.6 with rbenv install 3.3.6, then ran rbenv global 3.3.6. Those were the last two necessary instructions for my set up until I got to the instruction, “Whichever path you take, run the following command to see which version of Ruby you’re working with: ruby -v”. I ran that, expecting to see 3.3.6, but saw 2.6.10p210 (still). It turns out that after rbenv install... and rbenv global..., I needed to restart my terminal again.
Not sure if that goes for everyone, but it might be worth a mention to restart the terminal if the version expected isn’t the version that appears/reports.