If I attach a raven opposite to someone's season agenda, am I correct it assuming that the gold income is still affected

Attach Black Raven to your House card and discard all other Raven attachments from play. It is Summer. Each player takes 1 additional gold

i.e. If it cannot be summer, the text does not prevent you still getting 1 additional gold (or 1 less with the White Raven and a Cannot be WInter situation)

You are correct. The Seasons agenda only affects whether or not the opposite season is possible. All other text on the Raven would still apply.