This is an interesting one.
Given that the group was made up of newly ordained bhikkhus, who were clearly immature and undeveloped, it seems that they were not in the right frame of mind to listen to any instructions. The dismissal seemed to serve as a wake up call, which is not uncommon (see Ud 3.3 below). The four dangers mentioned sound like situations that cause the newly ordained to disrobe, so it is a sound teaching for those who’ve just gone forth.
The Buddha also seemed to use the dismissal as an exercise for his two chief disciples, clearly wanting to know how they would handle the group if the Buddha decided to no longer instruct them, which is interesting. Why would Ven. Sāriputta assume that he and Ven. Moggallāna would follow the Buddha in remaining passive, and why was Ven. Moggallāna so sure they would continue teaching them?
Take a look at the parallel Ud 3.3:
“Go away, mendicants, I dismiss you. You are not to stay in my presence.”
“Yes, sir,” replied those mendicants. They got up from their seats, bowed, and respectfully circled the Buddha, keeping him on their right. They set their lodgings in order and left, taking their bowls and robes. Traveling stage by stage in the land of the Vajjis, they arrived at the Vaggumudā River. They built leaf huts near the riverbank and there they entered the rainy season.
Then Venerable Yasoja, having entered the rainy season, addressed the mendicants: “Out of compassion, reverends, the Buddha dismissed us, wanting what’s best for us. Come, let us live in such a way that the Buddha would be pleased with us.” “Yes, reverend,” they replied. Then those mendicants, living alone, withdrawn, diligent, keen, and resolute, all realized the three knowledges in that same rainy season.
The key difference here is that these monks were clearly not immature and knew what they had to, so a dismissal with no follow-up instructions was sufficient.
Not sure if I’ve convinced you of anything, but hopefully this is a start.