I don't follow that reasoning. If the amendment is from 1976, I don't think it could be referring to the state of mind of people dealing with copyright in 1988 and the adhering to the Berne Convention. The problem is the provisions of section 405(a) are about how something can FAIL to be copyrighted due to the omission of the notice, the exception to the rule, a negative, yet the second half of the sentence treats it as if it's a positive, that it's about how to successfully copyright something without registering. If you have the proper notation, then you aren't subject to the provisions in section 405(a). Which doesn't make sense to the rest of the sentence.
I find the sentence badly worded, that it should either be "Registration is not a condition of copyright protection, subject to the exceptions in section 405(a)" or "Subject to the provisions of section 405(a), such registration is not a guarantee of copyright protection" depending on whether respectively they are trying to say that a work can be copyrighted without registering provided it has proper notation affixed or that just because something is registered, a work can be failed to be copyrighted if not followed through in applying the proper notations.
And, generally speaking, amendments and changes in the Law tend to not RESTORE copyright protection to public domain works (there are some exceptions, mostly foreign works). Thus, if something published in 1986 would be public domain by the 1976 rules but not by the 1988 rules, the change in the Law isn't necessarily going to change its current status, it's still going to be public domain though the current version of the Law would have protected it if it had been published two years later.
It's why this is a bit problematic. You need to know exactly what the law was when it was published (does it require registration, just proper notation or just being published afford it copyright protection) as well as what the law was when the time of renewal came up (does it fall under automatic renewals) and what it was at the time it came near the end of it's protection status (does it get the extra years and does if so, does it allow the original creators to sue for restoration of the rights).