Look at the

definition of when a first-order formula is true. "If A, then B" is true if A is false or B is true. So, "If 4 = 17 * 999, then 17 = 1 or 999 = 1" is true because the premise is false.

It's not that I suppose (i.e., assume); I just note that taking these values makes the statement after

true; therefore, the existential statement is true.

There does not have to be a causal connection between the premise and the conclusion in order for the (material) implication to be true. Ultimately, the reason for this is the definition given above. Philosophers may argue about other definitions.

Relevance logic has a different concept of implication.