Judging from experience, it shouldn’t be necessary to install the stock OS on Fairphone 3/3+. The e foundation gets the Fairphone firmware components to include in /e/OS directly from Fairphone via their official partnership.
/e/OS documentation in general is based on templates. When adapting the template default content to peculiarities of particular devices, deleting some unnecessary stuff can get forgotten.
Here’s a topic to clear this up: Fairphone - FP3 - FP3+ Documentation Suggestions