I thought that too, but the existence of highly affordable SDI-to-Thunderbolt adapters means the Mac Studio should still suffice unless you need a very large number of inputs... and I don't know how many multi-SDI PCIe cards there are.
AJA makes a couple of Thunderbolt 3 external SDI capture/output devices with 4x bidirectional SDI I/Os. If you want more than that you need multiple devices or a PCIe card. AJA and Blackmagic both make 8x SDI cards, as do a couple other vendors.
