The prohibition concerns separating the three rakahs by taslim. Sitting after the second rakah does not itself end the prayer. The Hanafi arrangement includes that sitting but concludes after the third rakah.
Opening interactive controls...
The Implication of “No Separation”
Witr Prayer: Three Rakahs, Qunut and Juristic Differences
The prohibition concerns separating the three rakahs by taslim. Sitting after the second rakah does not itself end the prayer. The Hanafi arrangement includes that sitting but concludes after the third rakah.