I get what you're saying. Can you conceive of a way that the story makes sense? Not everything is always clearly stated.

I can think of ways that make it make sense. For example, I don't think it's literally seeing cells — for that matter, individually seeing, isolating, and targeting millions of cells is entirely impossible in the real world. I think it would be closer to "sensing" or "searching", and maybe that takes intent and awareness into account, unlike many of our normal senses which often work without strict direction.

Maybe sensing cells in the air isn't easy without having a rough idea what to search for. A good analogy is those optical illusions that require focus to change the direction of rotation, or confusing perspectives that require focus to see what others see. If you can't see it because you don't know what to look for, then you can't be aware and focused on it, right? Maybe germ theory is new to their society. Maybe they aren't yet aware that humans are made up of cells, but are aware that infections are made up of many microscopic beings. Similarly, bacteria being in the air may be a blind spot to them, making them unable to focus effectively on them.

I imagine after this point in the story their society probably undergoes a drastic change. We saw a critical turning point in their world, and we didn't get to see the results. If they're able to draw life force from anything counted as "living" that they can focus on with their magical senses, then they've effectively become an immortal society with limitless magic.

There were some blockers along the way, such their senses requiring preexisting knowledge, their understanding of sacrifice being wrong, and the newness of germ theory, but they're on the path to immortality and infinite magic.

I guess I prefer to look at a story and say "how can this happen" rather than assume the least. Filling in the blanks is like a fun puzzle! I do think this is a super lopsided world and I think it would be hard for the author to continue stories in this world while keeping the same setting and atmosphere. They'd have to invent some really wacky rules to prevent insane power creep and massive societal shifts.