This passage from Megilah Point by Point Daf 22 seems to be incorrect as upon looking up the Rashis, Rashi does not say anything about being answered or not. he only addresses the svara for Nef. Ap.:
"1. (R. Elazar): An important person may not fall on his face unless he is sure that he will be answered, like Yehoshua - "Va'Yomer Hash-m El Yehoshua Kum Lech..." (Rashi in Ta'anis (14b) says that Yehoshua was answered. Here he says that Yehoshua was not answered. Perhaps Rashi means, in this matter you will not be answered (Sefas Emes). Alternatively, we learn from Hash-m's rebuke that an important person may not fall on his face unless he will surely be answered.)"
I believe it is correct. There are two ways one could explain 'like Yehoshua':.
1) An important person may fall on his face only if he is sure that he will be answered, like it was proper for Yehoshua to do, for he was sure that he would be answered.
2) An important person may not fall on his face if he is unsure whether he will be answered, like it was improper for Yehoshua to do.
We must say either:
2A: Hash-m rebuked Yehoshu'a for falling at this time, for this prayer would not be answered, and this is a disgrace.
2B: Hash-m rebuked Yehoshu'a for falling at this time, even though this prayer would be answered, for Yehoshua was not sure that he would be answered.
Rashi in Ta'anis (14b DH Reshai) explains the first way. Rashi in Megilah (22b DH Lamah) explains like 2b.
I prefer 2A over 2B, and the Sefas Emes explains this way, and I gave this as the primary explanation. I mentioned that alternatively, we could explain like 2B.
Pesach Feldman