The attitude of doubt adopted in the situation of receiving an unsigned letter is used to justify an attitude of skepticism toward reading statements from anonymous sources in newspapers—two situations presented as analogous.

The columnist doesn’t argue about different attitudes being adopted in two different situations. Rather, the columnist argues that the same attitude should be adopted in analogous situations.

The columnist doesn’t make any generalizations about a “type” of situation, only justifying one situation—doubting anonymous sources in newspapers—using another, analogous situation of doubting the contents of unsigned letters.

The columnist isn’t showing that any evidence is false, instead only arguing that it is reasonable to doubt statements from certain sources by comparing them to analogous sources that are usually doubted—not shown to be false.

The columnist doesn’t conclude that any information is likely to be false, only concluding that skepticism about some information is justified.