Might be better in it’s own topic, but I’d say I’ve never heard that advice before. Who are you hearing this from?
From searching around for anything like this, the best explanation I can find is people thinking about the calories and saying that the carbs in Half and Half are only slightly more than Heavy Whipping Cream anyway (that kinda depends on portions and what you are comparing).
At 15ml, Half and Half is nearly twice the carbs of Heavy Whipping cream (0.71g vs 0.41g, respectively). Neither is huge of course, so you could get away with either depending on how much you take. But, if you take a lot, that can make a difference. Some are still thinking the calorie disparity is a big deal which has an even wider margin, but since the calories come from fat I’m not as concerned. I’m often taking it wanting to get the extra calories that will tell me I’ve had enough anyway. (Disclaimer: I’m currently loosely testing out upping my intake actually to see if I’ll have some corresponding metabolism increase without putting on weight, or at least not long term weight, but I’d still give the same advice even when I was not doing this).
Alternative idea: there may also be some confusion due to language cropping up, as some English speaking countries (like the U.K.) have different terms for these items (Heavy Cream is “Double Cream” there), so somewhere along the lines there may be some mistranslation. That’s just a guess though not knowing who is saying this advice.