Does describing modern (metric) units of measurement as more precise and accurate than the units of measurement during the time of the Prophet, peace and blessings be upon him, constitute a disparagement of him or his Sunnah? And what is the ruling on one who says this without intending mockery or disparagement?

Your question stems from intrusive thoughts (waswasah) concerning disbelief (kufr). The cure for this is to disregard it, pay no attention to it, and stop asking about it. Describing modern units of measurement as more precise than older ones is not an act of disbelief (kufr) or a disparagement of the Prophet (peace be upon him) or his Sunnah.