Your claim above is "False" the event of fleeing to the mountains takes place "After" Daniel's future AOD, this starts the "Future" (Great Tribulation) Jesus returns (Immediately) "After" the "Future" great tribulation
(Immediately After The Tribulation Of Those Days)
Luke 21:21 and Matthew 24:16 is the exact same event of those fleeing (Judaea/Jerusalem)
(Items 1-4 Below Are Future Events Unfulfilled)
1.) Daniel's AOD
2.) Inhabitants flee Judaea/Jerusalem
3.) Great Tribulation Starts
4.) Jesus returns immediately after the tribulation
Luke 21:21KJV
21
Then let them which are in Judaea flee to the mountains; and let them which are in the midst of it depart out; and let not them that are in the countries enter thereinto.
Matthew 24:15-30KJV
15
When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand

16
Then let them which be in Judaea flee into the mountains:
17 Let him which is on the housetop not come down to take any thing out of his house:
18 Neither let him which is in the field return back to take his clothes.
19 And woe unto them that are with child, and to them that give suck in those days!
20 But pray ye that your flight be not in the winter, neither on the sabbath day:
21
For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be.
22 And except those days should be shortened, there should no flesh be saved: but for the elect's sake those days shall be shortened.
23 Then if any man shall say unto you, Lo, here is Christ, or there; believe it not.
24 For there shall arise false Christs, and false prophets, and shall shew great signs and wonders; insomuch that, if it were possible, they shall deceive the very elect.
25 Behold, I have told you before.
26 Wherefore if they shall say unto you, Behold, he is in the desert; go not forth: behold, he is in the secret chambers; believe it not.
27 For as the lightning cometh out of the east, and shineth even unto the west; so shall also the coming of the Son of man be.
28 For wheresoever the carcase is, there will the eagles be gathered together.
29
Immediately after the tribulation of those days shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken:
30 And then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and
they shall see the Son of man coming in the clouds of heaven with power and great glory.