The difference between a regular fast refreshable mview and an insert-only mview is - as you said - that an insert-only refreshable mview can only be fast refreshed after an insert statemenet and not after any other DML operation (such as delete & update)
I assume that the logic behind the restriction is that when you update an existing value Oracle has no way of knowing, by the mlog table alone, what is the new max (it will have to keep some sort of rank to do that).
Regarding the capabilities table - this is odd. Check this page - this did the same test but in their example they got
Capable of:
REFRESH_FAST
REFRESH_FAST_AFTER_INSERT
Not Capable of:
REFRESH_FAST_AFTER_ONETAB_DML
AMT_SUM
SUM(expr) without COUNT(expr)
REFRESH_FAST_AFTER_ONETAB_DML
COUNT(*) is not present in the select list
REFRESH_FAST_AFTER_ANY_DML
see the reason why REFRESH_FAST_AFTER_ONETAB_DML is disabled
Have you tried performing a fast refresh after an update?