Actually this is not true. The statute says as follows for that penalty:
765.30(2)
(2) The following may be fined not more than $10,000 or imprisoned for not more than 9 months or both:
765.30(2)(a)

(a)
Penalty for false statement. Any person who in any affidavit or statement made under
s. 765.02 (2),
765.09 or
765.11, willfully and falsely swears, or who procures another to swear falsely in regard to any material fact relating to the competency of either or both of the parties applying for a marriage license, or as to the ages of such parties, if minors, or who falsely pretends to be the parent or guardian having authority to give consent to the marriage of such minor.
765.30(2)(b)

(b)
Penalty for unlawful issuance of marriage license. Any county clerk who knowingly issues a marriage license contrary to or in violation of this chapter.
765.30(2)(c)

(c)
Penalty for false solemnization of marriage. Any person, not being duly authorized by the laws of this state, who intentionally undertakes to solemnize a marriage in this state; or any person who intentionally participates in or in any way aids or abets any false or fictitious marriage.
The previous section to this only says out of state weddings not recognized in Wisconsin are null and void in Wisconsin. It does not say anything about criminal penalties.