Calendar Sync blocks out time on your calendar when other folks look to your calendar for availability.
For example, if someone uses the "Find a Time" feature in Google Calendar to find time with you on your work calendar, Calendar Sync can automatically keep your personal calendar events blocked on that calendar to prevent them from booking over personal events.
Connected Calendars allows Reclaim to consider your availability from multiple calendars in order to block time for Habits, Tasks, Smart 1:1s etc., but others will not see this time as blocked out. For example, you might select your personal calendar as a Connected Calendar so your Tasks and Habits get booked around both your work and personal time, but that won't stop folks from creating events on your work calendar during an event on your personal calendar.
The short of it is: if you care about being overbooked within Google Workspace, use Calendar Sync. If you don't care about being overbooked, and you just want Reclaim to consider your availability across multiple schedules, use Connected Calendars.