Yes claiming that the "Great Tribulation" seen in Matthew 24:21 began during the Lord's time on earth is 100% preterist, as it claims the "Beginning" has been fulfilled when it hasn't
Please keep in mind that "beginning" does not mean consumation.
Sin began in the Garden from the Serpent to Eve to Adam = this was the BEGINNING of TRIBULATION upon the world
Sin is Tribulation
Sin always causes Tribulation
Sin/Tribulation began in Genesis and increased to the point where God had to Destroy every living thing save Noah/8 souls.
Likewise, Tribulation escalated after JESUS Rose from the Dead and Ascended and the Baptism of the Holy Spirit was poured out on Pentecost.
Then Peter stood up with the Eleven, lifted up his voice, and addressed the crowd: “Men of Judea and all who dwell in Jerusalem, let this be known to you, and listen carefully to my words.
15These men are not drunk, as you suppose. It is only the third hour of the day!
16No,
this is what was spoken by the prophet Joel:
17‘In the last days, God says,
I will pour out My Spirit on all people.
Your sons and daughters will prophesy,
your young men will see visions,
your old men will dream dreams.
18Even on My menservants and maidservants
I will pour out My Spirit in those days,
and they will prophesy.
19I will show wonders in the heavens above
and signs on the earth below,
blood and fire and billows of smoke.
20 The sun will be turned to darkness,
and the moon to blood,
before the coming of the great and glorious Day of the Lord.
21And everyone who calls on the name of the Lord will be saved.’
JESUS spoke of this in Matthew ch24 and HE Says that the last Sin that brings the world into Great Tribulation is the AoD of Daniel.
The AoD of Daniel is defined in Revelation which culminates with the Mark of the Beast = Revelation 14:9-13
Then a third angel followed them, saying with a loud voice, “If anyone worships the beast and his image, and receives
his mark on his forehead or on his hand,
10he himself shall also drink of the wine of the wrath of God, which is poured out full strength into the cup of His indignation. He shall be tormented with fire and brimstone in the presence of the holy angels and in the presence of the Lamb.
11And the smoke of their torment ascends forever and ever; and they have no rest day or night, who worship the beast and his image, and whoever receives the mark of his name.”
12Here is the patience of the saints; here
are those who keep the commandments of God and the faith of Jesus.
13Then I heard a voice from heaven saying to me, “Write: ‘Blessed
are the dead who die in the Lord from now on.’ ”
“Yes,” says the Spirit, “that they may rest from their labors, and their works follow them.”