> user can't touch files in directory. Ext4.
I double checked. Sorry the previous example was not good. To reproduce the issue, you have to create another directory inside the top one. Here is a working example:
# terminal A
su
mkdir /opt/experiment/
chown aristo:aristo /opt/experiment
mkdir /opt/experiment/apple
chown aristo:aristo /opt/experiment/apple
# terminal B,
whoami # aristo
cd /opt/experiment/apple
touch aaa # OK
# terminal A
chown root:root /opt/experiment
chmod 700 /opt/experiment
# terminal B
pwd # Gives /opt/experiment/apple
touch bbb # OK bbb is created
cd /opt/experiment/apple # Gives permission denied
# new terminal C
cd /opt/experiment/apple # Denied
touch /opt/experiment/apple/ccc # Denied
Note that, after chmod 700, in terminal B you can still create files, although you cannot cd into apple.