Incorrect. Although statistically you may have improved the ANC's chance of winning (though I'm not convinced) not voting is not the same as voting a party in to represent you in a government you don't approve of.
Mathematical proof that not voting favours the ANC more than any other party:
does vote
A) 10 000 000 voters
ANC: 6 000 000 = 60%
DA: 1 500 000 = 15%
COPE: 1 500 000 = 15%
IFP: 350 000 = 3,5%
FF: 250 000 = 2,5%
ID: 200 000 = 2%
rest: 200 000 = 2%
does not vote
B) 9 999 999 voters
ANC: 6 000 000 = 60.000006000000600000060000006%
DA: 1 500 000 = 15.0000015000001500000150000015%
COPE: 1 499 999 = 14.9999914999991499999149999915%
IFP: 350 000 = 3.50000035000003500000350000035%
FF: 250 000 = 2.50000025000002500000250000025%
ID: 200 000 = 2.0000002000000200000020000002%
rest: 200 000 = 2.0000002000000200000020000002%
(B) - (A) = the difference
In scenario B, except for Cope, the parties receive slightly more of the vote:
1) ANC: 0.000006000000600000060000006% more
2) DA: 0.0000015000001500000150000015% more
3) COPE: 0.0000085000008500000850000085% less
4) IFP: 0.00000035000003500000350000035% more
5) FF: 0.00000025000002500000250000025% more
6) ID: 0.0000002000000200000020000002% more
rest: 0.0000002000000200000020000002% more
The number of votes the ANC gets stays the same, but the
proportion of votes the ANC gets is higher.
ANC gains 0.000006000000600000060000006%
DA gains 0.0000015000001500000150000015%
The ANC's
proportion gain in scenario B is
higher than all the other parties' proportion gain. It is from the total proportion of votes that the 400 parliament seats are allocated.
{eg: ANC's gain - DA's gain
0.000006000000600000060000006% - 0.0000015000001500000150000015% = ANC receives 0.0000045000004500000450000045
% more than the DA}