He is asking you to source your claims. That is not an unreasonable request. If you provided sources, maybe you could backup your argument and change some opinions.
For example, if I wanted to state "the Western Roman army deserted upon the death of Aetius in 454 A.D." Few people would take that as face value. I'd have to back it up, for example with the Given Translation of Priscus via John of Antioch fr. 259 on Page 127 of Given's Translation (his fr. 69 of Priscus), and his second fragment again preserved through John of Antioch (fr. 71). And then I'd have to provide my interpretation: fr. 69 states Aetius was "General of the legions" when discussing his death in 454, and fr. 71 states that Gaiseric noted Petronius Maximus had "no noteworthy forces" at his disposal when he invaded Italy in 455 and sacked Rome.
I think that effectively conveys my point, although granted I'm working with far more limited sources than a 15th or 16th century historian would. Such extent of interpretation may therefore not be necessary. Providing sources makes an argument legitimate.