How could “Apollos” teach about Jesus accurately if he did not know another baptism than the baptism of John?

The long answer (requires speculation):
There is so little about Apollos in the scriptures that it is impossible to tell exactly what 
Apollos taught about Jesus. As an educated Jew, “with a thorough knowledge of the 
scriptures” (Acts 18.24), we could say for sure that Apollos knew the Old Testament 
well. He would’ve have been very familiar with the Messianic promises contained in the 
Old Testament. Those promises would include (among others) that the Messiah would: 
1. be a descendant of Abraham (Ge 12.3; Ac 3.25-26);
2. be a descendant of Judah (Ge 49.10); 
3. be a prophet like Moses (De 18.15-19; Ac 3.22-23);
4. be born of a virgin (Is 7.14)
5. make the blind see (Is 35.5-6)
6. be crucified (Psalm 22)
7. be raised from the dead (Ps 16.10-11; Ac 1.3, 2.32). 
A core teaching of Apollos would’ve included the hoped for suffering servant of Isaiah 
53 a passage, when looking back through the lens of the New Testament, makes it easy 
for the faithful to see Jesus.
He would have also taught through the prophet Malachi chapter 3, verse 1: “I will send 
my messenger, who will prepare the way before me. Then suddenly the Lord you are 
seeking will come to his temple; the messenger of the covenant, whom you desire, will 
come,” says the Lord Almighty (Mal 3.1) another passage, when looking back through 
the lens of the New Testament, makes it easy for the faithful to see John the Baptist.
“Knowing the baptism of John” may then be a euphemism for knowing the Old 
Testament completely. 
We can only speculate that Apollos having been “instructed in the way of the Lord” and 
knowing only the “baptism of John” (Ac 18.25) means that a disciple of John the Baptist, 
who left Jerusalem just after Jesus began his public ministry, both baptized and formally 
taught Apollos in Alexandria. In this scenario, neither would have had knowledge of the 
death and resurrection of Jesus, and the coming of the Holy Spirit. 
Short answer (the answer I like):
While Apollos was teaching the Messiah from the Old Testament, our writer Luke (and 
Priscilla and Aquila) received his teaching from a fuller New Testament point of view. 
As such, while Apollos taught, they were (rightfully) hearing Jesus. Luke wrote it in that 
point of view. 
– Steve Johnson