Lemang is basically glutinous rice that has been sitting in coconut milk inside a bamboo tube over a fire, so the santan gets fully absorbed into every grain. The result is dense, sticky, slightly fatty rice that is genuinely delicious. At 290 calories for 2 slices, the rice itself is not outrageous, but the carb count is high (glutinous rice hits harder than regular rice gram for gram) and there is almost no protein to speak of. If you are tracking, note that these numbers are a ballpark estimate as there is very little hard nutritional data published for lemang specifically, and fat absorption from coconut milk varies depending on how long it is roasted and the ratio used.
The number that actually matters is what you put next to it. Two slices of plain lemang at 290 cal is fine. Add 2 tablespoons of serunding kelapa and you are at 380. Add a full piece of beef rendang and you are at 490 calories in one plate. The rendang is worth it calorie-wise because it adds real protein, but the serunding is basically just fat and coconut with very little upside. Portion is everything here: one slice of lemang (roughly 90g) cuts you to about 145 calories, which leaves way more room for rendang.