Indiscreet discrete mathematician checking in. If they said "exactly" we'd have a real problem. Instead, "precisely" in this context means "human eye cannot distinguish from exact value at a stone's throw."

Yes, physicists and engineers hate me, why do you ask?

"It's precisely 21 cm."

"Is it 21 cm?"

"No."

I expect the non-technical author/editor was playing the telephone game and originally wanted to emphasize that the frequency is always the same value, not that the hydrogen emissions frequency is related by arbitrary factors of 9192631770.000 Hz, 1/299792458.000 seconds, and then exactly 21.000/100.000 to the caesium-133 frequency.

so the claim is inaccurate by 1mm and missing precision data. I'd call it inaccurate and imprecise XD

The exact opposite would be ”imprecise” or ”inaccurate”

Accuracy and precision are orthogonal concepts. “Approximately 0 light years” is accurate but not precise.