This is a clarification of a regulation that may have been misunderstood in the past.
If the signature matches that held on file then the vote can be counted, whether the signature crosses the seal of the envelope or not.
Florida does require mailed votes to be signed by the voter. The signature is put on the envelope. And the envelope is printed in such a way that it is possible and common for people to sign over the seal (which also adds a degree of security, making it harder for a person to modify the vote after it has been sealed)
Because of this, the supervisor of elections in individual counties may have thought that the signature must cross the seal. But this is not the case. The clause clarifies this fact. If the signature matches then the envelope should be opened and processed whether or not the signature crosses the seal.
In logic from P ⇒ Q it does not follow that ¬P ⇒ ¬Q, nor Q ⇒ P (but (P ⇒ Q) is equivalant to (¬Q ⇒ ¬P))