The Mekhilta methodically eliminates every possible misunderstanding about how the Torah was given at Sinai. Each wrong assumption is raised and then demolished by a specific verse.
One might think the Torah was given at night — in darkness, in secret, when no one could see what was happening. The Torah therefore states (Exodus 19:16): "And it was on the third day, when it was morning." Morning. Daylight. Full visibility. God did not whisper His revelation in the dark. He delivered it in the blazing light of day, when every eye could witness what was occurring.
One might think the Torah was given in silence — transmitted telepathically, or through quiet meditation, or in the still small voice that Elijah would later hear at Horeb. The Torah therefore states (Exodus 19:16): "And there were thunders and lightnings." The giving of the Torah was the loudest event in human history. Thunder crashed. Lightning split the sky. The mountain itself shook. Silence was the last thing present at Sinai.
But perhaps the thunders were merely atmospheric noise — impressive but unintelligible, sound without meaning. One might think the people did not actually hear or understand what was happening. The Torah therefore states (Exodus 20:15): "And all the people saw the thunders and the lightnings." They saw sound. They perceived the thunder with their eyes. And the Psalmist confirms the overwhelming nature of the experience (Psalms 29:4): "The voice of the Lord in majesty." The Torah was given in broad daylight, with deafening thunder, and the people both heard and saw every moment of it.