I don't think pulling out the number 6 separately from the constant factor results in an apples-to-apples comparison with other operations with logarithmic complexity.

The 32-way tree operations have asymptomic complexity 6 * C_1 * log(n) = C_2 * log(n)

Operations on a balanced binary tree have asymptotic complexity C_3 * log(n) = 6 * C_4 * log(n).

The only difference between the two data structures is the actual values of the constants.

I think the Scala people have a valid point that logarithmic complexity may be as good (or nearly as good) as constant-time in practice. The precise way the claim is formulated is wrong and abuses big-O analysis.

A more correct argument is to choose a practical upper bound on log(n). E.g. maybe 64. Then multiply that by the constant factor (not playing any tricks with splitting out 6 *). If that number is always good enough if practice, then you don't need to worry about your operation being logarithmic.