From code here:

https://chromium.git.corp.google.com/chromiumos/third_party/autotest/+/f35c065c2901dc00a5760fcb580e684a722b0077/client/common_lib/cros/chrome.py#87

It says arc instance doesn't get started by default.

But actually it will start even we don't pass "-a" flag to autologin.py

Then arc will begin to crash (see b/67637786) because there is no play store installed and then crash the whole chrome.