You are correct that the walled garden contributes to security by creating "another hurdle" for attackers to overcome.

Nobody is arguing that Apple's approach isn't more secure; they are arguing that the tradeoff (an additional layer of security vs. the right to install the software of your choice in a computing device you paid for) is not worth.

There are countless examples of this. Requiring everyone to strip completely naked to get on an airplane would absolutely act as an additional hurdle for a person with nefarious intent, but we don't do that because we acknowledge that there must be a fundamental tradeoff between our safety and our freedoms.