"The emphasis is on logical necessity, not observation."

Logical necessity means:

If the premises are true and the reasoning is valid, the conclusion must be true.

Observation means:

We have seen many examples supporting the conclusion.

Example:

Observation:

Every swan I have seen is white.

That suggests all swans are white.

But discovering one black swan disproves it.

By contrast:

If you prove a theorem correctly in mathematics, one new example cannot overturn it.

That is why mathematical certainty differs from empirical certainty.