When I create a layer, select it, then choose the Text Tool, it creates a new layer. I believe this is a bug, as it should go into the layer I selected. It's frequently a frustration for me, only in that I have to go and delete and rename layers. But I do consider it a bug.
If you have a completely blank layer selected, that can be replaced with the type layer. If you click on an existing type layer, you can edit that type layer to add more text. But you can't just add editable type into an existing layer that has other (non-text) contents in it.