Ignore the word heat. A heat map isn't specifically related to actual heat, both in this game and when used as a general mapping term. A heat map is a map that shows how dense/congested/etc. some statistic is in an area. For example, a heat map of population could show a change in color based on density. In the game, the heat map relates to activity and not to heat. So doing actions like forging something or using a cement mixer or firing a gun will increase this activity level. Now, that activity can include things like sound or actual heat as well. Anything that would indicate to something that there is a person nearby.
Now, that just explains why it affects things regardless of temperature. But it doesn't solve your actual problem of not liking screamers. I think there is the possibility they'll include an option for screamers when they add the 50-100 new game options they are talking about. And I think that's a good option to include. I don't care about screamers as they are easy to deal with. Sure, they can be annoying at times, but it takes almost no time to deal with them and then I don't have to worry about them for a long time, so it isn't a big deal to me. But I know a lot of people hate them, so having the option to change their spawn rates and even disable them entirely would be a good option to include.