SC Book, Page 127, A question about basics

[AZ679]

[Sorry if I have asked many questions recently; I have reviewed the Manhattan SC book a few times and have encountered these problems which I preferred to ask them all at once]

Manhattan SC BOOk, 5th edition, page 127, bottoam of the page:

(I know this question is about basics, but I have problem with it)

"Like swimming, skiing is great exercise"

Is it correct to use 'a' before great here?
Is there a change in meaning?
Why the original sentence is correct without 'a'? It seems more natural to me to use 'a' before 'great exercise', as here we are talking about a kind of exercise, and using exercise as a countable noun here.

Could you please explain this issue?

[RonPurewal]

"a"/"an"/"the" is not tested on this exam at all. so, it's a non-issue.