No, Frequently Visited is a completely different animal. You have no direct control of what goes in or when. With Top Sites, you had total control of was available with a couple of clicks, then the site of interest was then available with one click until you decided otherwise(deleted the link).
The Frequently Visited feature is controlled by the software itself (I think SIRI is involved in some manner). I frequently check an 'underground dot com' radar page, often a couple times a day, yet this page NEVER appears in Frequently Visited. Since using Safari 14, I have visited my credit card website once to make an account payment, that damn webpage showed up immediately in this Frequently Visited section of Start Page. Hmmm, that's rather odd behavior.
No sir, Top Sites and Frequently Visited are NOT interchangeable in functional operation at the user experience level. Whether or not Frequently Visited will be the new Top Sites remains to be determined, however, that is not the case right now.