Yes you are correct. If I have a numerator and denominator that are the same then there is no difference. Sample size for the subgroups will impact the width of CI's. As for, "CI width of (RR 4.9; 95% CI 0.99-9.00)" if you were taking the hard rule that if "1" was in the CI you would fail to reject the null then it would be not be 'significant'. Of note, this was considered a hard rule in the past, but contemporaneously people are starting to acknowledge that the study would provide some evidence that there could be some relationship going on.

So if you are dealing with a ratio the CI cant include ''1", if that is your null value. If you are dealing with a difference (e.g. rate difference) then the CI cant include "0", if that is you null value. And in some cases, if desired, you can set your null to whatever value your want to test, like in a one-sample t-test.