The fact that a government contract was canceled does not, in and of itself, result in a partial plan termination. The PPT is determined starting with a "rebuttable presumption" that all participant terminations are involuntary. You can then prove otherwise, if you can, based on facts and circumstances.
Were all these people already 100% vested in all accounts? If so, the PPT has no real effect.
There are few absolutes in this arena - see the Matz case. But in most circumstances, if your turnover rate was at least 20% involuntary terminations, then I agree, it is a PPT, barring unusual or egregious circumstances. If, as you say, it was less than 20%, then it would generally not be a PPT.
Your situation seems pretty straightforward. Don't forget to take into account the CAA relief, if applicable, with your March 31, 2021 participant count when determining PPT status for 2020.