Si53156 is internal termination for maximum integration which the output buffers in Si53156 are push-pull buffers that integrate the 33-ohms series resistance and the 50 ohms to ground. Please refer to AN781 Alternative Output Termination for Si5211x, Si5213x, Si5214x, Si5216x,Si522xx, Si5310x, Si5311x, and Si5315x PCIe Clock Generator and Buffer Families
For inputs, they are not self-biased. The buffer also expects a DC coupling HCSL input. A bias voltage needs to be provided if the input is AC coupled.