The reference in the text doesn't state anything more than that it was a snake, not that it was a Blue Krait. We don't even get to see Harry's reaction, be it familiarity or perceiving it as novel.

The snake is described as "gleaming soft white" and "silver", which fits with the description of a patronus. And as it doesn't match the description of Quirrelmort's animagus snake "bright green and intricately banded in white and blue", it is clearly not Quirrelmort's animagus form.

While I assign a much higher probability that we just saw Drako's patronus, we can't rule out the possibility that it was someone else's patronus, including Quirrelmort, even though I see those odds as being exceptionally low.