Peace
وينذر الذين قالوا اتخذ الله ولدا
Furthermore, [this divine writ is meant] to warn all those who assert, "God has taken unto Himself a son."
18:5
ما لهم به من علم ولا لءابائهم كبرت كلمة تخرج من أفوههم إن يقولون إلا كذبا
No knowledge whatever have they of Him, and neither had their forefathers: dreadful -is this saying that comes out of their mouths, [and] nothing but falsehood do they utter!
i think in these ayats the judeo-christians with the son-god belife are mentioned it is not titled to be shirk whereas it is called "dreadful" or "falsehood".
But further in this sura someone is mentioned with a monotheistic belife but his Actions and sayings are titled shirk.
18:35
ودخل جنته وهو ظالم لنفسه قال ما أظن أن تبيد هذه أبدا
And having [thus] sinned against himself, he entered his garden, saying, "I do not think that this will ever perish!
18:36
وما أظن الساعة قائمة ولئن رددت إلى ربى لأجدن خيرا منها منقلبا
And neither do I think that the Last Hour will ever come. But even if [it should come, and] I am brought before my Sustainer, I will surely find something even better than this as [my last] resort!"
18:37
قال له صاحبه وهو يحاوره أكفرت بالذى خلقك من تراب ثم من نطفة ثم سوىك رجلا
And his friend answered him in the course of their argument: "Wilt thou blaspheme against Him who has created thee out of dust, and then out of a drop of sperm, and in the end has fashioned thee into a [complete] man?
18:38
لكنا هو الله ربى ولا أشرك بربى أحدا
But as for myself, [I. know that] He is God, my Sustainer; and I cannot attribute divine powers to any but my Sustainer."
In last vers the word shirk is used .
The Story of Gardenowner goes on in sura 68:17-68:33 and is showing which deeds are shirk .
peace