How are the electricity retailers able to charge such low prices? Has SP been overcharging us all this while?

They are charging lower price to get more market shares. The electricity tariffs are set by the EMA, theres nothing SP can do about it as well.

Do note those electricity retailers are buying from SP. They do not own any power stations or power generating assets, transmission and infrastructure.

SP owns power stations, solar panels and other assets. These would entail maintainence costs, upgrading, future investments in power assets as well. People like to complain about SP making huge profits but think about it, if 10 years down, we need a new power station, where does the money come from if SP has no profits?
When you do not own or need to build or maintain any assets, you can definitely charge lower.