What's missing from that article is the detail that (as everyone reminds me) AJ has said there will be an F cabin on the Project Sunrise aircraft.
So the QF configuration will be 4 class not 3 class, and should have even fewer than the 300-320 the article envisages.
The A350-1000 cabin is only 5 inches wider than the B787-9 cabin so that won't make a huge difference and I can't see QF doing 10 abreast in Y on this aircraft.
It is 11 metres longer. It seems to me this doesn't offer the chance to do much more than add an F cabin of 12-14 seats similar to the A380 F cabin.
Adding that to the 789's 42 J, 28 PE & 166 Y would give you about 250 seats.
If you made the F cabin much smaller - say as few as 6 seats - then being optimistic you could increase the J seats by about 16 or PE by 21 or Y by 27.
Or some combination thereof, which might be influenced by doors, bulkhead, galley and toilet placement.
Of course extra toilets and possibly galley space dedicated to first would eat into the room for extra seating.
But on these rough calculations you would be looking at between 250 (14 F), 258 (6F, 58J) or at most 269 seats if you have an F cabin at all.
You could squeeze more in but that would be inconsistent with AJ's stated "premium heavy" aproach.
At 270 or fewer seats the range of the A350-1000 would be looking pretty good!