[T.B. Yoits]

They didn’t use the whole SV40, but a 72-nucleotide segment of the DNA (a promoter gene) got in.

It didn't just "get in". Someone put it there.

[grey_whiskers]

Might’ve been piss-poor Quality Control.
The method they used for detecting DNA segments had a lower length detection limit of ~100 nucleotide chains.

Of course, “mistakes on purpose” do happen...

[TStro]

And it was impossible to separate
Your talking particles measured in nanometers - billionths of a meter.