To answer my own question, as expected blood oxygen SPO2 sensor is there and working, but they're kind of avoiding mentioning it (not a word in the keynote) and it probably works a bit different under the hood so it can exploit a loophole in the patent infringement case.Blood Oximeter???
Head to the apple watch page, scroll to "more features to empower your health", keep clicking to go right, and blood oxygen is the LAST feature: