Why is AC coupling needed between PHY and Fiber transceiver?
"PECL compatible" doesn't mean the part actually has a PECL topology in the output driver circuit, it just means it can be used with PECL parts.

In fact Fig 9 of the DP83620 datasheet strongly implies the DP83620 outputs are not PECL because true PECL would require a pull-down resistor on the output in order to function at all.

Since the DP83620 datasheet never as far as I can tell gives an equivalent circuit for the outputs, you can't really do much other than follow the recommended application circuit in Fig. 9.