Learn English – Is this superlative degree because of usage of most or is it positive degree

The terms most rare, rarest, more rare, and rarer have all been in use for centuries. Here is the Ngram chart for the years 1800–2005 (search result matches go back to the 1500s):

The chart suggests that rarest (red line) and rarer (yellow line) have been more popular (not popularer) forms than most rare (blue line) and more rare (green line) for at least 150 years. Nevertheless, given their consistent occurrence in published works over so many years, I wouldn't argue that any of the four forms is wrong.

Perhaps one reason for the persistence of the more rare and most rare forms is that the contrary forms are less rare and least rare. Also, to the extent that comparatives of rare arise in contradistinction to comparatives of common, more rare and most rare may benefit from the relative popularity of more common (green line) and most common (blue line) as against commoner (yellow line) and commonest (red line):

Here, commoner is the least common of the four terms despite (or perhaps partly because of) the fact that it has an identical noun form.

Our rotary telephone is the least frequently used device in our house. 
Our rotary telephone is the device that is least frequently used in our house. 

In the first sentence, it is not the superlative adverb that is used with the definitive article "the".  In both versions of this sentence above, the article is a dependent of a noun.  In the original, the participial phrase "least frequently used" is another dependent of the same noun.  Moving the participial phrase into its own clause demonstrates that the article remains tied to the word "device". 

A participial phrase like this, including the superlative adverb construction, can be found standing without a visible noun to modify.  These can be understood as elliptical constructions: 

Of all the devices in our house, the rotary phone is the least frequently used [device].