You still have full access to those seasonal activities. Your own fault if you forgot to claim *past* season rewards before their prospective season ended.
The issue here is they advertise that you can claim it all through their website but it's requesting that you purchase it all over again so you be the one to explain that to me and the justification behind it bud.
Easy. No where does it say you can claim past rewards from the season pass out of season. They actually say everywhere every season to make sure you claim ALL pass rewards before the season ends. You still have access to the season activities. Those won't leave until Final Shape.