That is correct. Don't make too much over this. This is just the way UL is adding a benefit to RCAF membership with the priority seating. The deadline was set so that they can then start assigning seats based on the forms they have received.
You should still purchase using the forms they have even if you don't meet the RCAF or season ticket requirements ... they will assign the best seats they have remaining based on postmark date for your entry (or email date if emailed).
They will continue selling tickets until they don't have anymore - that won't end on the 6th.