Signal never syncs old messages on secondary clients for security reasons.
Also, considering that linking requires access to your existing device I don't see an issue with that. Moxie himself considered usability to be more important than tinfoil hat-level crypto because large-scale adoption is what enables security.
The limitation is that only message history from the past 45 days would be synced. If this has changed recently to allow syncing all message history, I’d be thrilled!