And just to emphasize, "the preferred form of the work for making modifications to it" is a direct quote from GPLv3, under which the Linux kernel is licensed.
<https://www.gnu.org/licenses/gpl-3.0.en.html>
If GrapheneOS are finding the Google Drive option inadequate, they might choose to invoke Section 6 clause b of the GPL "Convey the object code in, or embodied in, a physical product (including a physical distribution medium)".
Note that AOSP as a whole is covered under the Apache 2.0 licence, which permits but does not obligate third-party distribution of source. That is, it's not a "copyleft" licence in the sense of the GPL.
Googles notes on why BSD/MIT/Apache licences are preferred are ... informative.
<https://source.android.com/docs/setup/contribute/licenses>