They're not second guessing it, they're reading what Motor Voter says. The argument being pushed by the minority of SCOTUS is that even though the law says this, Ohio still should have used some other means that would have meant less people being removed from the voter rolls because reasons.
https://www.law.cornell.edu/uscode/text/52/20507
1. A state shall not remove the name of a registrant from the official list of eligible voters in elections for Federal office on the ground that the registrant has changed residence unless the registrant—
(a) confirms in writing that the registrant has changed residence to a place outside the registrar's jurisdiction in which the registrant is registered; or
(b)
has failed to respond to a notice described in paragraph (2); and
(ii) has not voted or appeared to vote (and, if necessary, correct the registrar’s record of the registrant’s address) in an election during the period beginning on the date of the notice and ending on the day after the date of the second general election for Federal office that occurs after the date of the notice.
2. A notice is described in this paragraph if it is a postage prepaid and pre-addressed return card, sent by forwardable mail, on which the registrant may state his or her current address, together with a notice to the following effect:
(a) If the registrant did not change his or her residence, or changed residence but remained in the , the registrant should return the card not later than the time provided for mail registration under subsection (a)(1)(B).
If the card is not returned, affirmation or confirmation of the registrant’s address may be required before the registrant is permitted to vote in a Federal election during the period beginning on the date of the notice and ending on the day after the date of the second general election for Federal office that occurs after the date of the notice, and if the registrant does not vote in an election during that period the registrant’s name will be removed from the list of eligible voters.
(b) If the registrant has changed residence to a place outside the registrar's jurisdiction in which the registrant is registered, information concerning how the registrant can continue to be eligible to vote.
(3) A voting registrar shall correct an official list of eligible voters in elections for Federal office in accordance with change of residence information obtained in conformance with this subsection.
And from the CNN news story:
Thus Ohio complies with Motor Voter Act bolded section as described above. Case dismissed.