Which will split the vote.. Not down the middle I agree, but unless no one votes for him at all, he will cost the SNP votes.
I think you're wrong, as this isn't the English system.
Just look at the arithmetic. Alba could split the anti-SNP vote, and increase SNP control.
Alba candidates aren't contesting the 73 Constituency seats, as they have no chance there.
They are standing for the 56 regional AMS seats (8 regions with 7 seats each, awarded in proportion to overall party share of the vote).
The 129 Members of the Scottish Parliament are 61 SNP – 30 Cons - 23 Labour - 5 Green - 5 Lib-Dem - 3 Independent MSPs – 1 'Reform UK'.
The SNP have 57 of the 73 constituency seats, but have only 4 of the 56 AMS seats.
The Conservatives won 7 constituency seats, their other 23 are all AMS regional seats.
Labour have 3 constituency seats with 20 AMS regional seats. The 5 Greens all have AMS regional seats. The Lib-Dems have 4 constituency seats, 1 AMS seat.
The whole point of AMS proportional representation is to check the imbalances of 'the first past the post' constituency system, provide effective opposition, and give minority parties a say. If Alba get any votes at all from the AMS share they will weaken the opposition, and strengthen the call for independence which is their avowed intention.