Yes, you're right, sequencers use digital logic to advance steps, and have done so since the 1960s. But what's distinctive about Cessna's approach is that he discretized the waveform itself, encoding its shape as a table of amplitude values. That's clearly digital synthesis in the sense of constructing signals from numbers. Also, his implementation of pitch tracking was computational. But I think ultimately the distintinction between digital and analog is moot: even on analog systems, we are thinking digitally when we use on-off gates or when we use a sample and hold to transform continuous voltages into stepped ones.