Hi, author here. Yes, my point with that sentence was that the average (non-technical) user (to which Signal is marketed btw), is not going to check signatures. Google has root on the phone, the user is using their app store to install the Signal app that comes up in the store, and basically Google has full control over this, and the user would be none the wiser.

Of course, us more technical inclined people could then check the signature, or compare the apk with one built from the official sources, to see the difference and complain about it.

But between those things is a time frame where this is possible.