The key line in the article is:
Given the large number of tickets already contracted, however, the remaining seven- to 12,000 would be prioritised for the RFU’s affiliated clubs and key stakeholders.
Emphasis mine. It's not that they are making a decision now to prioritise corporate over club, it's that they were already locked into long term contracts with the corporate hospitality firms, and there are far fewer tickets left to distribute.
It's got some interesting implications for rugby at the Stoop though - if Twickenham can support 35-45,000 - roughly 42%-55% of capacity - then if similar percentages apply to the Stoop that would mean we could see 6,000-8,000 at the Stoop. That would probably allow at least the STHs to come to the games - though we might have to sit in unaccustomed parts of the ground!