What that Battery University article claims depends on various factors. A properly designed charging circuit does not continue trying to push charge into a fully charged battery. It is technically true that fully charged is more stressful for a battery, but then so too is running the battery down. A charge cycle is also a complete discharge and recharge, so it takes many small topping up charges to make up one charge cycle. So I think it is complicated and really not easy to determine what real world scenario actually works best.
The thing is those older keyboard designs are far more robust. They keep working even when dust gets in, which it inevitably will.