Why is foreshadowing used in lines 8 and 12 of sonnet 18?

Most of the 'Fair Youth' sonnets (approximately 1 - 126) are addressed to an unmarried young man. Their usual argument is: 'you are so beautiful that you need to get married, and raise children; beautiful people who never marry leave nothing of their beauty behind on earth after they die'.

But Sonnet 18 breaks this pattern, since it says that the Fair Youth's beauty will live on long after his death in Shakespeare's poem.

Since Shakespeare's poem is referencing the far distant future (long after both the Fair Youth and Shakespeare have died) - it is inevitable that the poem will talk about future time (this isn't really foreshadowing, the poem is directly referencing a future state beyond itself).

But as usual, Shakespeare is messing with us. Sonnet 18 talks about beauty, and preserves the beauty it talks about for all time (or at least, for well over 400 years). But we don't know who the Fair Youth is. So the poem is being less than honest with us.