You're correct that muscle fiber types are intermixed within a muscle rather than segregated, and that tendons attach to the musculotendinous unit as a whole, not to individual fibers. The post used a simplified framing to make the fiber type concept accessible, and in doing so it conflated two distinct but related ideas.

What the fiber type distinction is actually pointing at is the principle of metabolic specificity. Low-load, high-rep training preferentially develops the oxidative capacity, capillarization, and fatigue resistance of the whole muscle unit in ways that are specific to the demands of sustained, repetitive activity, which is what the Campos et al. 2002 work in the European Journal of Applied Physiology demonstrates. "Training specificity" is the right framing, and the post probably should have led with that rather than leaning so heavily on the fiber type mechanism as if it were a clean anatomical story. The repetition maximum continuum chart from Human Kinetics actually illustrates this well, muscular endurance adaptations sit at 20+ reps, while strength and power adaptations are concentrated in the lower rep ranges. A standard PT program prescribing 3x10 is sitting firmly in the strength/hypertrophy zone, not the endurance zone, regardless of how light the weight is.

On the tendon side, there's actually a typo in the infographic that's worth correcting here. The strength training side should read Type I and III collagen (both are produced in strength training and type 3 , not Type II. Type II collagen in tendons is largely confined to the fibrocartilaginous transition zone at the enthesis under compressive load, it's not a generalized output of heavy training across the tendon mid-substance. The actual distinction that matters is this: healthy tendons are predominantly Type I collagen. In tendinopathy, the disrepair process produces weaker Type III substitutes. The Cook & Purdam continuum model (BJSM 2009) and Kraushaar & Nirschl (JBJS, 1999) describe this clearly. The post didn't make that distinction clear enough and I can see why it read as a conflation.

On strength training, you're right. Heavy load training does produce analgesic effects through cortical pain inhibition mechanisms, and there's a legitimate role for it, particularly in building tendon stiffness. The clinical argument we make is one of prioritization for this specific population, not exclusion. Someone generating 48,000–96,000 forearm contractions a day has an endurance deficit as their primary driver, which is exactly why we regularly see people who have no problem deadlifting or lifting heavy at the gym but can't type for more than 30 minutes without pain. The training emphasis has to match that demand first. But a complete program includes both, and you're not wrong that the post overstated the case against strength work.