But it does mean that those two ships effectively doubled Elite's income for the last six months
No, not really. It's probably not even the case that they doubled the ARX income specifically.
Comparing the information on the various pages of the investor presentation, the post-Ascendancy boost is visible on the revenue graph, but is so much smaller than the relative boost on the Elite PDLC graph that almost all of that boost must have been from new base-game sales.
PDLC income in November compared with March was almost seven times higher on a monthly basis [1], but total revenue for ED was only about doubled comparing the two months. The only way that's possible is if PDLC income makes up a very small fraction of the total. [2]
If base-game sales income was identical in March and November (which the text I think rules out) that still limits PDLC income to making up only ~50% of November's income and ~15% of March's income. Even a modest 50% rise in base-game income comparing March and November would limit PDLC income to under 10% of March's income and still only about 25% of November's.
(And PDLC income, in this context, is going to include Odyssey purchases, and purchases of existing cosmetics by some of these new accounts)
I expect the ships have all paid for their own development (which is all they
really needed to do) but the big boost to general income has come from things like Powerplay and the battle for Sol attracting (directly or indirectly) lots of new players (or new alts) to the game.
[1] The second graph showing ED PDLC income doesn't have a scale, which I expect is
very deliberate to avoid making it too obvious how small the absolute values involved are.
[2] Similarly, PDLC income in May and August was over three times that of March, but the
total Elite Dangerous income for March, May and August was essentially indistinguishable for all three months. The graph is low-resolution enough that you can't get too much out of that, but again it does mean that the very obvious spike in November/December can't have been mainly PDLC income.