What is the point of taking a Guaranteed Payment?
I agree with you that for individual tax purposes, both the distributive share of the income and the guaranteed payments are subject to Self-Employment Taxes!
But, sometimes in partnerships there is one partner who is a manager or operating partner who demands that he gets a fixed salary, or a payment for actively managing or operating the business. For this reason, this partner gets a guaranteed payment before any share of profit or loss distribution occurs.
The Net profit that results after deducting the normal and necessary business expenses along with Guaranteed Payments, is then shared amongst the share of the profits based on the partnership agreement.
Now, if there are 2 partners and both are operating the business and putting in equal time and effort, there could be case made that there be no guaranteed payments at all, and then you could allow the flow of this income to be subject to self-employment taxes for both the partners.