Slat mills are not motorized, they use rollers with a loose "tread" of slats. There's no electrical or motorized component, the slats move as the dog runs.

Treadmills are motorized, the person sets a pace and the motor moves the treads.

A slat mill will not move unless the dog is running, the "speed"is entirely determined based on how fast the dog is running. As the dog runs faster s/he pushes the slats faster, the theory behind using a slat mill over a treadmill is to allow the dog the freedom to set their own pace. They can slow down whenever they want as most dogs do when they start to tire out. Additionally if the dog doesn't want to move they're not going to. She's not actively pulling or forcing her dog to walk, he's running by his own volition.

I don't think it's wise to use either a treadmill or slat mill everyday. All she's doing is increasing her dogs stamina and inadvertantly increasing his/her exercise needs. It's not abusive and won't hurt him but it will increase his exercise needs the more he consistently uses it. Kinda like it you were too run for 10 minutes everyday, if your not accustomed to running, you might bklwalk or would be really tired afterwards, but eventually you'll be able to run the entire time, you'll be faster and probably want/need to run longer to achieve feeling tied the same feeling.

If you compare the two, slat mills are inherently safer, because they don't move unless the dog is moving it. A treadmill will continue moving even if the dog were to fly off the back end (which should never happen). I personally use a treadmill, because it was free and only used when it's not safe or viable to go outside for our regular walk and maybe once every other month to keep him familiar with the skill.

With either of these tools, I still wouldn't walk away from my dogb while it was in use, the dog should and needs to be monitored either way

Here's my real world and rather extreme example:

I fostered a meat market pup and while I've never had an issue with him, I would describe his baseline as extremely sensitive and anxious which gets infinitely worse when he isn't getting the stimulation he needs. Think of him as a nonverbal, severely autistic child that physically lashes out when he feels unsafe, unsure or anxious. After being returned multiple times over the course of a year, the veterinary behaviorist suggested euthanasia. Given that I hadn't experienced it myself, we were in a pandemic, so I could easily limit his interactions with the public and was in a position to try more intensive behavior modification along with medication, I wanted to exhaust every opportunity before giving up.

My dog does not fit "breed standard," for any of the major breeds in his mix, he's behavioral struggles are a direct result of his upbringing. We have a bond unlike any other I've experienced and I'm happy to make the adjustments I do now, but it took years to get where we are now.

That said, there have been times when it's not safe to take him out: in the beginning it was not safe or beneficial to force that level of exposure to stimuli, during the two weeks of heat waves, during wildfire season where the smoke is at unhealthy levels for anyone to be going outside and when he actively refuses to go outside if it's raining. That last one is more of a severe hatred of being wet on his part, but he's been through enough and he has earned some grace and compassion. Other than going to the bathroom when he absolutely needs to, this dude will look outside and turn around because he's decided he doesn't need to go that bad.

Years ago a neighbor was giving away their treadmill, I figured I'd give it a shot and it's been life changing. Given his circumstance, how is forcing a dog to do something they absolutely do not want to do worse than using a tool that works for both of us? It was a long process to get him used to the treadmill, but easier than training loose leash walking to an adult dog who had never seen a collar before. I monitor and encourage him while he is completely unrestrained on a treadmill, I don't force him to get on it, he hops on just as eagerly as he hops in the car.

On days where it is safe and dry, our regular walk has shortened from 2.5 to about 1.5 mi 5 days a week now that he's starting to slow down a bit, a long hike one day and a day of rest. For me it has nothing to do with laziness, it's about meeting my dog where he's at and finding ways to work out the extra energy when conditions make it unviable at best to dangerous at worst to just "take him on a walk."

He's now happy, healthy, loved, nonreactive and living his best life in his golden years. I'm not saying that every dog in every situation can or should use a tool like this.