A says: 他自从遇到车祸以后就不开车了。
B says: 还真是，我以为他不喜欢开车呢。
'还真是' here is similar to "It is actually true" in English (agree/affirm to A's statement)
In Cantonese, we would say "噉又係喎"
噉(this) | 又(also) | 係(true) | 喎(final particle: indicate 'agree/affirm')
maybe "真係吖", very roughly means "really?"
We say this when we don't think about something true in our daily life and suddenly, somebody around us mentioned it and we are like:" yeah! it is actually true! I never thought about it before!" Literally means: "It is actually true".
还真是 = 原來係噉
原來 >>> the unexpected original/real (reason)
係噉 >>> is this (reason told by A)
还真是 = 噉至真
噉 >>> this (reason told by A)
至真 >>> is the unexpected real (reason)