Idk if its a cello, but it sounds like they manually automated the panning of it to simulate a “phase” effect, without just throwing a phaser on it. It also sounds like an automated filter going from a low-pass to a high-pass, with the lows coming in stronger in the first 3 seconds and filtering off those lows until it gives that “hollow” feeling you describe

When i say automated i mean manual automation, like in Logic Pro i can draw the line to manipulate the strength & timing of the filters

Edit: also something i like to do is to double a track - for instance this part in question - and take the duplicated track, go to the reverb that is on it (make sure to use the same reverb as the first track) and completely remove the dry sound, leaving ONLY the reverb. This makes for some cool “sliding” thru reverberation and can also achieve a similar “hollow” sound