It is simple. If the answer to the question: "How would it be if everybody acted like me?" is "It would be worse", this means you are acting in a way that makes society worse for your own advantage.
That is, when he asks "What if everybody lied?", what he's getting at isn't that it'd suck if you couldn't even ask for the time and get the right answer, but that it'd destroy the concept of truth, and with that what does "lying" even mean?
Kant answered the specific scenario of "What if the Gestapo wants to know if you're hiding any Jews?" (his actual scenario was with a murderer looking for a victim, since Nazis weren't a thing yet), and said that yup, you do not lie, because to him the morality of an action is unrelated to its likely consequences.
If there were a fund (assume a spherical, frictionless, voluntary society) that was the only way to feed the hungry, everyone should donate to it. Unfortunately it's set up by an evil game master, and one in ten thousand donations is answered with a bullet to the spine.
Of course the categorical imperative says we should do it anyway. I wouldn't. You wouldn't. Rational defection on bad payoff matrices is correct behavior.
Just to be abundantly clear, I offered no advice. I simply offered a clarification to the comment I replied to, with my assumption of what they meant.