“No god was formed before me” can be read by a Latter-day Saint as consistent with the Father being unformed within the sphere relevant to us, while leaving open a prior sphere. The verse isn’t decisive.

That reading requires the text to mean something narrower than it says, and no single verse is decisive against a determined reading. The cumulative force of Isaiah 40 to 48, in which God repeatedly claims to be alone, first and last, and incomparable, with none before and none after, is the argument, which is why the claim says repeatedly. A sphere-limited reading has to be applied to every one of those statements, and at some point the exception is doing more work than the text.