Thanks for the question,

I guess the author goes more this way

--the smallest exact number integer data type that can't support a negative number

declare @bit bit

set @bit = -1

Command(s) completed successfully.

declare @tinyint tinyint

set @tinyint = -1

Msg 220, Level 16, State 2, Line 5

Arithmetic overflow error for data type tinyint, value = -1.

so certainly bit datatype can get a negative value which will be transformed to 0 or 1.