Purists would say that, but are they accurate? A variation in speed from faster to slower, done on purpose, is a form of fartlek. Just because some people want to argue it is not, does not make them right.
Since another poster used Wikipedia to explain a fartlek, I will also do so. Here is what the first three paragraphs say. They appear to say what I am saying.
Fartlek is a middle and long-distance runner's training approach developed in the late 1930s by Swedish Olympian Gösta Holmér.[1] It has been described as a relatively unscientific blending of continuous training (e.g., long slow distance training), with its steady pace of moderate-high intensity aerobic intensity,[2] and interval training, with its “spacing of more intense exercise and rest intervals.”[3][4] Simply stated, in its widely adapted contemporary forms, Fartlek training is alternating periods of faster and slower running, often over natural terrain, including both “level and hilly terrain.”[3][5][6] While Fartlek training is generally associated with running, it can be incorporated into almost any kind of exercise. The variable intensities and the continuous nature of the exercise stresses both the aerobic and anaerobic parts of the runner's physiology. It differs from traditional interval training by being less structured.[7][8][9] An example of its more modern manifestations in the training of serious runners is found in Mona Fartlek, named for Australian distance runner Steve Moneghetti, devised by Olympian Chris Wardlaw. This training style injects speed into a 20 minute session, pairing alternating periods of effort and recovery: 90 seconds on, 90 seconds off (performed twice), then 60 seconds on-then-off, and 30 seconds on-then-off, and 15 seconds on-then-off (each of these performed four times), generally, with intensity (pace) increasing as the effort period shortens, with the specifics determined by coach and athlete.
The bolded parts agree with what I am saying. They are LESS structured, meaning there is some form of structure. Further, one version has specific times as the rates for each alternating segment, which is exactly what I said.
Blah, blah, blah...
Try leaving your watch at home and doing a real Fartlek session.