Yes it is…but in reality it is a huge misconception…it has been proven this is a myth. Getting cut is a function of dropping body fat…which is mostly diet. You can burn more calories doing cardio but as you posted you will lose muscle doing this…that is where HIIT training comes in. A very basic example of HIIT is to sprint for 20 seconds and walk for 30-40 seconds and repeat 8-12 times…will target fat directly and not muscle. Here is a little writeup I found on the myth of high reps = getting cut…there is a ton of info on this subject.
Myth #5: Doing high reps with lower weight will give definition, and low reps with heavy weight will bulk you up.
The fact is whether you do high reps low weight or low reps heavy weight, your muscles will get toned. Heavy weight low reps will not necessarily bulk you up, and low weight high reps will definitely not get you cut (give you definition). A matter of fact, getting cut and being able to see definition has nothing to do with how you perform your lifts or how much weight you are using; it has to do with how much fat you have covering your muscles. Losing fat is the only way to get definition and cut. Most women do not have to worry about bulking up because no matter what they do, they do not have the quantities of hormones needed. Have you noticed how power lifters are smaller than most bodybuilders, and yet are much stronger? Power lifters work mostly with heavy weights low reps and although they do gain muscle, they don’t gain as much as bodybuilders.
For more information on this subject read the section in my book “Live Your Way Thin,” on weight training.
Here is the link where there are a bunch of other myths
http://www.stavrosm.com/The7BiggestMythsoftheWeightLossIndustry.html