I'm finding conflicting information here as well. The statute says "shall" but follows, to the letter, section 103 of the Katrina Emergency Tax Relief Act of 2005:
https://www.congress.gov/bill/109th-congress/house-bill/3768/text
(FWIW, the coronavirus-related distribution provisions of the CARES Act track section 101 of KETRA very closely.)
Notice 2005-92 gave guidance on implementing both sections, which I imagine is similar to what we will get under the CARES Act.
The notice doesn't outright say "under KETRA, the loan extension is optional," but section 5(b) of the notice very strongly implies that it is optional (e.g., "Thus, an employer is permitted to choose to allow this delay in loan repayments under its plan with respect to a qualified individual, and, as a result, there will not be a deemed distribution to the individual under § 72(p)" and "If a qualified employer plan suspends loan repayments during the suspension period, the suspension will not cause the loan to be deemed distributed even if, due solely to the suspension, the term of the loan is extended beyond five years" and "If an employer, under its plan, chooses to permit a suspension period that is less than the suspension period described above, the employer is permitted to extend subsequently the suspension period, but not beyond December 31, 2006." See also the example in section 5(b).