Does the guide/driver stay in the car while you mooch round the attraction and with a fully guided tour they come round the attraction with you?
That's about it. You would probably meet up with a local guide at each place. I note that on the partially-guided tours the price of admission to the attractions is not included either, whereas when it's fully-guided the guide goes to the ticket office and buys the tickets.
If you think about it, the driver could be driving any size of vehicle. You wouldn't expect the driver of a 50-seat coach to also get out and start giving guided tours of various monuments. Plus, driving in Sri Lanka is pretty full-on. Our driver/guide (just the 2 of us in a normal car) sometimes talked to us on the cruising bits, but there were sections where you didn't want to be asking him how the Sinhalese alphabet worked as he was overtaking a tuk-tuk with a truck bearing down on him.
If you are in a bigger group, you might also find that they mix in people from different countries in the group and perhaps there would be two or three guides, each speaking different languages. But my guess is that your party would likely be small. We were delighted to just be the 2 of us in a car, rather than 4 or 6 in a minivan, but if being on your own would be a showstopper then you would want to get some assurances from the tour operator.
You can also expect several stops at restaurants or craft shops where the driver will be getting a commission on what you buy, or at least a free lunch for taking you there. Sri Lankan food can be very, very spicy, so unless you are the kind of person who orders a phaal when you're out for a curry in the UK, you are probably safest letting the driver take you to their recommended place, where you will generally find a buffet of not-too-spicy local dishes. It's expensive by local standards but cheap by yours, which I guess is what's known as a win-win.