Well, you don't exactly sign in, at least in the conventional way, on an Android phone for a Google Account.
You have to use Settings, Google, and add any accounts you wish to use from the device there. It would be highly unusual for anyone to have any accounts other than their own on a given Android smartphone. These devices are considered to be single user devices and don't have multiple user accounts like PCs do that create entirely dedicated environments for multiple users. I have three Google Accounts and sync e-mail from all of them on my smartphone. It is quite possible to use multiple Google Accounts on a single smartphone, but within a single user environment. This presumes this single user should have access to all of those Google Accounts.
If you were to lose your phone you would most likely be using a tablet or PC to log in to your Google Account, not someone else's smartphone, and that's what they assume. I never accessed Android Lost from anything other than a PC when I needed to locate my phone which had gone MIA.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
. . . the presumption of innocence, while essential in the legal realm, does not mean the elimination of common sense outside it. The willing suspension of disbelief has its limits, or should.
~ Ruth Marcus, November 10, 2017, in Washington Post article, Bannon is right: It’s no coincidence The Post broke the Moore story