Employee contribution to 401(k) is nearly always pre-tax, so there is taxable conversion. Employer match is always pre-tax, so there is tax on conversion. Earnings were sheltered, so there is tax on conversion.

But an employer plan can provide for Roth 401(k), and only the employee can contribute to that, and it is post-tax. That makes it already taxed. Converted that to Roth is more like a Rollover, since it is from Roth to Roth. It would not be taxable.