If all you set is the ScreenTipHeader to text and don't have a ScreenTipDescription, ScreenTipFooter, ScreenTipHelpUri etc. set, it should render the ScreenTip like a ToolTip. If any of those ancillary properties are set, it will render more like a ScreenTip with bold header.
You can test this with this code that I updated in the Ribbon "Standalone Controls" QuickStart (I changed ScreenTipDescription to ScreenTipHeader):
<ribbon:SplitButton ImageSourceLarge="/Images/Icons/Save32.png" Label="Large SplitButton" VariantSize="Large" ScreenTipHeader="This SplitButton has a VariantSize of Large.">
In that code it looks like a regular ToolTip.
If I then update it to add a ScreenTipHelpUri attribute, I start to see the "Press F1 for help" text too.