Hi, welcome to the forum!
What the storage overview refers to, is the ability to set modification time, such that it can be used to later reliably compare timestamps of source and destination.
The cloud storage system supports setting modification times on objects. If it does then this enables a using the modification times as part of the sync. If not then only the size will be checked by default, though the MD5SUM can be checked with the --checksum flag.
All cloud storage systems support some kind of date on the object and these will be set when transferring from the cloud storage system.
(https://rclone.org/overview/#modtime).
If size is not enough to detect changes in your use case, the --checksum flag could be added. Though, Seafile does not support that either, it seems - it is listed without any hash algorithms in the overview.
In another thread it was mentioned that it is possible to use Seafile with Webdav and S3 backends, so perhaps you can make it work with either of these? You will not be able to access "files in an encrypted library" (Beta Testing: Seafile backend), so I don't know how that affects you..