Technically, you're correct. It should be "a range of [plural noun] is", because "range" is singular. In practice, both are used, but "is" is more common. You can see usage statistics of "range of antibiotics is/are" here: https://tinyurl.com/ydysoaay You can also see usage statistics of "range of subjects is/are" here: https://tinyurl.com/ycqjvru6 As you can see, "is" and "are" are both used, but "is" is used more. I would stick with "is".

This use of "are" is a mistake. As you see, the last word of the noun phrase "a broad range of antibiotics" is plural. So, the person who wrote this text just made a mistake. We are all human after all. Usually such mistakes occur with noun phrases having more words.