In the Thomas Hardy poem, "The Convergence of the Twain", why is the ship described as "prepared for a sinister mate" in line 19?

In the poem, "The Convergence of the Twain", published in
1915, and commemorating the sinking of the luxury ocean liner
Titanic three years before, Thomas Hardy gives free reign to his
deterministic view of reality. Thus, after mocking the pride and vanity of
the 'unsinkable' ship in the first six stanzas of the 11-stanza poem, in the seventh
Hardy avers that it was an "Immanent Will that...prepared a sinister mate for her...A
Shape of Ice" (lines 18-21), referring, of course, to the iceberg which - sitting across
the sealane - sent the Titanic to the bottom of the North Atlantic
with a loss of 1200 persons; significantly, mention of this is missing from the poem. In
these lines Hardy reveals his creed, if it can be called that. His 'god' is an
impersonal, dispassionate "Immanent Will", capitalized in the poem to show its
implacable power. Hardy later employs a synonym for it - calling it "the Spinner of the
Years (line 31) - thus, bringing his view closer to the Fates of Greek mythology, those
three sisters who spun out the inescapable threads of destiny. As Titanic took shape, so
Hardy's thesis goes, "in shadowy silent distance grew the Iceberg too", until the
'twain' encountered one another in their predestined, ill-fated collision. Reinforcing
his view that no beneficent Mind governs the universe, Hardy depicts the destruction of
Titanic, identified as she, by the iceberg, qualified mostly with
male terms, as a kind of violation, using such words as   “ravish”, “mate”, “intimate
welding”, and “consummation”. Finally, a sadness pervades the poem, but especially in
last five stanzas. It is not a sadness over the loss of life, but over the realization
that one is trapped in a meaningless universe, the plaything of forces frighteningly
real, but unseen, like the hidden bulk of an iceberg.