Snape joining Hogwarts

[Hitesh]

No: HPFGUIDX 96419

In PoA (Page 261,Chapter 18, Bloomsbury Paperback) Lupin mentions
Snape. Sirius interrupts him and says 

<Quote>

'Snape?'..."What's Snape got to do with it?'
'He's here, Sirius,' said Lupin heavily. 'He's teaching here as well.'

<End quote>

Doesn't this exchange imply that Sirius did not know that Snape had
ever become a Hogwarts teacher? Sirius went to Azkaban in early
November 1981. So Snape would have become a Hogwarts professor only
after Voldemort's downfall. I wonder why there is some controversy
about whether Snape joined Hogwarts before or after the fall of Voldemort.

On the other hand there is mention of Snape in the Shack before.

<Quote from page 253, chapter 17)>

'Ages,' Hermione whispered. 'Since I did Professor Snape's essay...'

<End quote>

Probably Sirius wasn't listening then.