Perfect!
This is true . . . listen to chords from the bottom up. In the case of the chords above: If a pianist saw those C2(omit3)/over-a-bass-note symbols, they would know to play CDG, or GCD, or DGC on top of the chord, plus the added bass note in the bottom. Only those notes. The other symbols would suggest the pianist was free to add other options outside of that specific voicing. I see both ways, depending on what the composer was thinking.
The slash chord voicings reflect what Wayne Naus, in his book "Beyond Functional Harmony," calls "ambiguous," "hybrid," or "incomplete" voicings. Sometimes those specific sounds are what the composer wants.
Thanks, @Whousedtoplay, and @Fredrik E. Nilsen — Maria is a very down-to-earth person, generous and supportive with the musicians who work with her. Great composer and bandleader. I played in her band a couple of times in NYC when she was first getting her big band together. She's worked with us at the WDR a handful of times over the years—always fantastic! That video you posted was from quite a few years ago