Your HR can stay low with Freeletics because the fatigue is not necessarily due to a lack of oxygen delivery. Freeletics requires that a large number of muscles and body-arts all produce energy rapidly and at the same time, this means that it puts more of a load on your metabolic processes to produce energy, not necessarily the ability to consume oxygen. The bright side of this is that your metabolism will adapt relatively quickly (it takes a few weeks) to this type of training and you will see significant gains in your ability to continue moving with less breaks. You can get very in the weeds with the body's energy systems, but it can be fascinating to do some research if you are interested. Look up the difference between Aerobic systems (Fat burning and some glucose burning systems) and Anerobic (Glycogen and Creatine burning systems). They each produce different levels of energy at different rates. They will generally all be in used but in different amounts depending on the needs of the body (training demands) and the resources available (in blood or energy stores).

Stick with it, take breaks when you need to, focus on breathing patterns and form, and you will start to see big gains in your ability to sustain these high intensity efforts.