In the example sentences for 確かに we have:
彼のうちは音楽一家らしいが、確かに楽がうまい。
I heard he is from a musical household, but he is certainly skilled with an instrument.
I am confused as why が is translated into “but” here. It seems that “being skilled with an instrument” is a logical conclusion from “being from a musical household”, rather than an unexpected outcome.
I see in the dictionary が has the entry:
used after an introductory remark or explanation
which I believe if more appropriate here?
I think its expressing surprise
like “well I did hear that, but he really is skilled, moreso than I would have thought from that alone”
But I also think your interpretation is correct
edit: it could also be read as “I had heard he was from a musical family, but having not confirmed it for myself I didn’t think much of it, but now I see that he is certainly skilled with an instrument”
By the way, is there a way to similarly have a introductory remark in English?
Under the influence of Japanese I’m always tempted to use “but” for it.
I would lean towards using:
Ah, so he’s from a musical household. Indeed he’s skilled with an instrument.
I dont think that fits the use of “rashii” though 
I think the “but” here is being more as a conjunction/transition between the two clauses, in the same way as quoted below.
- が, like ‘but’ in English, combines two sentences which express contrastive ideas. However, が is much weaker than ‘but’ in that it is sometimes used simply to combine two sentences for stylistic reasons even if those two sentences do not represent contrastive ideas. For example, the が in (1) is used simply as a transition word to connect two sentences.
(1)
a. パーティーをしますが来ませんか。
We’ll have a party. Wouldn’t you like to come?
b. 田中さんは私のうちにも時々来ますがおもしろい人ですね。
Mr. Tanaka sometimes comes to my place, too. He is an interesting person, isn’t he?
From the Dictionary of Basic Japanese Grammar, page 121.