This is picky, but I expected a just a list of dependencies. The output implies that updates were attempted. (Hopefully none would have been updated even if some were out of date).
[I] ZSwiftPM> swift package update --dry-run
Updating https://github.com/swiftlang/swift-syntax.git
Updated https://github.com/swiftlang/swift-syntax.git (0.35s)
Computing version for https://github.com/swiftlang/swift-syntax.git
Computed https://github.com/swiftlang/swift-syntax.git at 602.0.0 (0.40s)
0 dependencies have changed.
Here is the help.
[I] ZSwiftPM> swift package update --help
OVERVIEW: Update package dependencies.
USAGE: swift package update [--dry-run] [<packages> ...]
ARGUMENTS:
<packages> The packages to update.
OPTIONS:
-n, --dry-run Display the list of dependencies that can be updated.
--version Show the version.
-h, -help, --help Show help information.
Suggestion: "The packages to update." should be something like "The packages to update (default - all packages)." or "The packages to update (default - current directory)." Depending on what really happens.