Your link is a commentary that begins by claiming Tiamat was 1st and Apsu was 2nd. Here is the text:
1 When the heavens above did not exist,
2 And earth beneath had not come into being —
3 There was Apsû, the first in order, their begetter,
4 And demiurge Tia-mat, who gave birth to them all;
5
They had mingled their waters together
6 Before meadow-land had coalesced and reed-bed was to he found —
7 When not one of the gods had been formed
8 Or had come into being, when no destinies had been decreed,
9 The gods were created within them:
10 Lah(mu and Lah(amu were formed and came into being.
11 While they grew and increased in stature
12 Anšar and Kišar, who excelled them, were created.
13 They prolonged their days, they multiplied their years.
14
Anu, their son, could rival his fathers.
15 Anu, the son, equalled Anšar,
16 And Anu begat Nudimmud, his own equal.
The metaphor was the marshlands where runoff met the saltwater of the Gulf. Tiamat was the salt water and the Apsu was the fresh water. You posted a link a while ago claiming the Sun might have supplied the Earth with some of its water, hydrogen attaching to dust grains. Fresh water and salt water mixed and 2 gods formed between them followed by 4 more further from the Apsu (Sun).
Tiamat was a planet, Heaven and Earth came from it. A belief in other worlds permeates myth. These gods and their destinies are planets and their orbits.