The irony is the key: someone wary of extravagance (wasteful excess) was himself extremely profligate (recklessly wasteful) with paper and ink. The contrast between his professed caution and actual behavior is the point of the sentence. 'Probity' means honesty — unrelated to the extravagance/waste theme. 'Acquisitive' means wanting to acquire things, not specifically wasteful. Only extravagance...profligate maintains the ironic contrast perfectly.