They use the seatbelt to secure the child after 18kg. The isofix in those cases is just to secure the seat when it's empty, and to stop it from moving around when the child gets in and out of it.
It's isofix + harness which is limited to around 18kg (it's a bit higher to account for the weight of the seat itself). It's to do with what the isofix fittings themselves are actually rated to hold in a collision. A seatbelt of course is designed to hold an entire adult's weight. I don't know if isofix with top tether is rated higher, it seems like it could be, but I'm not an engineer so I don't know. Certainly all seats on the market which harness higher than 18kg (the next stage is 25kg) use the seatbelt to secure the seat in the car.
FWIW, intermittent wriggling in a seat is not a big problem with a seatbelt, because a child who leans out of a seatbelt then sits back in their seat will have the seatbelt adjust itself (as long as the HBB is designed well and the seatbelt isn't trapped, anyway). That's better than a harnessed seat with a child who can take their arms out because they tend not to want to put them back in, meaning you have to stop the car and do it for them, and if you can't stop straight away they are in an unsafe position for longer. Of course, it's more dangerous for the time they are leaning out of the seat, and there have been cases of children who have died or been injured because they were leaning out of a seat when the car crashed, but sometimes it's about judging the risk against other possible risks.
One more plus for high backed boosters which many parents don't think about - they can be used as a makeshift body board/stretcher to carry a child out of a catastrophic crash without moving their neck, meaning they can be taken to a safer place to be examined for injury. You can't do this with a harnessed seat - too heavy - and you certainly couldn't do it with a low booster.
Marzipan if you still have his red book, look in there to gain an idea of when he might reach 18kg based on the centile lines he's on or between. From 3 your normal bathroom scales are okay to check weight too, DS certainly always enjoyed standing on them!