Yes, this is true.
While the government allows only parents and legal guardians of girl children to open the Sukanya Samriddhi Yojana (SSY) accounts, the depositors toward those accounts can be different persons, i.e. is individuals other than the parents/legal guardians.
Thus, anyone can deposit amount in to the account. Do note, however, the tax-benefit under section 80C for the deposited amount can only be claimed by the parent or the legal guardian not by the depositor.