Amorite individual people did not live for 400 years while Israel was in Egypt. So God is not talking about individual Amorite but all the Amorites along with the other nations Israel is about to dispossess.
And there was also the NEPHILIM in the land. In genesis God destroyed all the corrupted people who had been corrupted by Nephilim, and now we see they have established their stronghold over the land Israel was to posses. This was a work of the enemy to oppose the prophecies God had given. In Genesis, it was about the coming Messiah to be born, who had to be pure, uncorrupted and have no part in Satan's physical  influence in the flesh  over the entire world, so only Noah was perfect in all his generations, (think DNA) the rest of mankind had been altered by the Nephilim into the seed of the serpent as the fallen angels had taken human women as wives and had children by them, corrupting all flesh.  This is not just spiritually corrupted, everything about all flesh had at that time corrupted itself from how God had made them. And the solution was a  global extinction of all flesh that breathed air to eliminate that satanic breeding. The same thing was happening again.
Exodus 23:23
For My Angel will go before you and bring you in to the 
Amorites and the Hittites and the Perizzites and the Canaanites and the Hivites and the Jebusites; and I will cut them off.
Exodus 33:2
And I will send 
My Angel before you, and I will drive out the Canaanite and the 
Amorite and the Hittite and the Perizzite and the Hivite and the Jebusite.
Numbers 21:34
Then the Lord said to Moses, “Do not fear him, for I have delivered him into your hand, with all his people and his land; and you shall do to him as you did to Sihon king of the 
Amorites, who dwelt at Heshbon.”
 
Numbers 22:2
Now Balak the son of Zippor saw all that Israel had done to the 
Amorites.
Numbers 32:39
And the children of Machir the son of Manasseh went to Gilead and took it, and dispossessed the 
Amorites who 
were in it.
Amos 2:9, Nephilim giants of large stature
“Yet 
it was I 
who destroyed the 
Amorite before them, Whose height 
was like the height of the cedars, And he 
was as strong as the oaks; Yet I destroyed his fruit above And his roots beneath.
Amos 2:10
Also 
it was I 
who brought you up from the land of Egypt, And led you forty years through the wilderness, To possess the land of the 
Amorite.
Numbers 13:32-33 New King James Version (NKJV)
32 And they gave the children of Israel a bad report of the land which they had spied out, saying, “The land through which we have gone as spies is a land that devours its inhabitants, and all the people whom we saw in it are men of great stature. 33 There we saw the [a]giants (the descendants of Anak came from the giants); and we were like grasshoppers in our own sight, and so we were in their sight.”
Footnotes:
Numbers 13:33 Heb. 
nephilim
etc...
Genesis 6:4
There were 
giants on the earth in those days, and also afterward, when the sons of God came in to the daughters of men and they bore 
children to them. Those 
were the mighty men who 
were of old, men of renown.
 
Numbers 13:33
There we saw the 
giants (the descendants of Anak came from the 
giants); and we were like grasshoppers in our own sight, and so we were in their sight.”
 
Deuteronomy 2:11
They were also regarded as 
giants, like the Anakim, but the Moabites call them Emim.
 
Deuteronomy 2:20
(That was also regarded as a land of 
giants; 
giants formerly dwelt there. But the Ammonites call them Zamzummim,
 
Deuteronomy 3:11
“For only Og king of Bashan remained of the remnant of the 
giants. Indeed his bedstead 
was an iron bedstead. (
Is it not in Rabbah of the people of Ammon?) Nine cubits 
is its length and four cubits its width, according to the standard cubit.
 
Deuteronomy 3:13
The rest of Gilead, and all Bashan, the kingdom of Og, I gave to half the tribe of Manasseh. (All the region of Argob, with all Bashan, was called the land of the 
giants.
 
Joshua 12:4
The other king was Og king of Bashan and his territory, 
who was of the remnant of the 
giants, who dwelt at Ashtaroth and at Edrei,
 
Joshua 13:12
all the kingdom of Og in Bashan, who reigned in Ashtaroth and Edrei, who remained of the remnant of the 
giants; for Moses had defeated and cast out these.