I don't think it's necessarily connected with modesty, although that could play a role in some circumstances.So, we are dealing with a modest historian here
Now it’s more or less clear. Still I’m not sure that I understand "aggregate of [significant] past events" correctly. Is it an enumeration of events with the dates? Something like this:As pointed out by valvs, "history" can mean either the aggregate (= sum) of past events or a chronological account of them. The former is of course unique, and hence is invariably accompanied by the definite article. The latter is usually (but not always) non-unique, thus necessitating the indefinite article.
"The History of Britain" (The word "history" is used here to mean "aggregate of [significant] past events".)
It’s logical, as any account or description of an event is nothing but somebody’s account/description of it. But look at this:"A History of Britain" ("History" is used here in the sense of "chronological account of past events".)