Yes it would, but you’ve got to remember that the function of a high backed booster is simply to lift your child up so that the seatbelt can restrain them correctly in the event of a collision. A high backed booster doesn’t have to fit solidly in a car with little or no movement like a seat for a younger child does.
So a non SL isofix would be sturdier? As for us it would not be used in another car.
We have two of these seat and I’d definitely recommend. Fab price, heat!
The SL part of the name stands for Soft Latch - this photo is a Kidfix SL (which I paid £76 for a couple of years ago and thought was a good deal, it was the cheapest I could find at the time) and you can see the isofix points are on a strap. They are not a rigid part of the seat. They can be pulled in completely so that you can use the seat in any car.
This uses ISOFIT which is a variation on isofix