State law requires that ballots must remain sealed until the commencement of tabulation. (Government Code Section 53755.5(b)(2)). State law also requires that:
During and after the tabulation, the ballots and, if applicable, the information used to determine the weight of each ballot, shall be treated as public records, as defined in [the California Public Records Act], subject to public disclosure and made available for inspection by any interested person. (Government Code Section 53755.5(b)(4)).
While property-owner mail ballot election for property-related fees are less common than registered voter elections, the California Supreme Court has specifically held that the California Constitution does not require secret ballots for this sort of property-owner mail ballot election. The City was aware of the election procedure required by State law, and flagged for property-owners the unusual aspect of this type of election in an effort to be transparent. The City printed on every ballot:
To ensure the privacy of your ballot prior to tabulation, please return it in the Ballot Return Envelope that was included with this ballot. This ballot becomes a public record once it is opened during tabulation.
It's worth noting that in 2014, the state adopted additional transparency practices for these types of elections due to some agencies abusing the process by trying to keep property owner ballot results secret. The result was amendments to the government code to require that these ballots be public records.