I gave it another try and it works

   asdf which node
   /Users/jmfayard/.asdf/installs/nodejs/19.0.0/bin/node
   
   which node
   /Users/jmfayard/.asdf/shims/node
   
    node --version
   v19.0.0
   
   asdf local nodejs 16.18.0
   
   node --version
   v16.18.0

I probably had read the documentation in the wrong order. Thanks for your message!