Hi buckethead ,
So the trade on the SD-Card interface is that if the SD-Card is to be allowed to be bootable, it needs to be connected to the same physical lines as the eMMC, which then necessitates that either eMMC or SD-Card can be used.
It is possible to put the SD-Card on a different interface connected to the Pi, but this results in the SD-Card not being bootable.
So the decision we made was to make the SD-Card bootable, as this gives folks the flexibility to flash a large SD-Card and boot from it - but this prevents eMMC and SD-Card being used at the same time. The alternate decision would be to make the SD Card usable at the same time as eMMC, but this would prevent the capability to boot from the SD-Card.
And all initial Pi compute module depthai all booted from SD-Card.
A better decision perhaps would have had 2 SD-Card slots so that a bootable uSD card could be used and also an uSD card that works while eMMC is present.
We can send you a Pi CM module without eMMC if you'd like so that you can use the uSD-card slot.