You missed my point, Ar...it wouldn't have been the Picard of the altered timeline, it still would have been the Picard of the normal timeline. From Guinan's 19th-century perspective, both are valid possible futures. The event that alters the timeline doesn't occur until the 24th century.
Thus when the timeline changes, Guinan knows that something's wrong, because suddenly she's in a different 24th century than the one that 19th-century Picard came from.
Thus when the timeline changes, Guinan knows that something's wrong, because suddenly she's in a different 24th century than the one that 19th-century Picard came from.