我让它工作，但右边的项目需要颠倒，为了保持顺序，这是由于float: right颠倒了元素的顺序。
基本上删除了flex容器，并使用float right/left来定位元素！
styles.scss

p-menubarsub {
  width: 100% !important;
  .p-menubar-root-list {
    display: block !important;
    list-style-type: none !important;
    margin: 0 !important;
    padding: 0 !important;
    overflow: hidden !important;
    & > li:nth-child(1) {
      float: left !important;
    }
    & > li:nth-child(2) {
      float: left !important;
    }
    & > li:nth-child(3) {
      float: right !important;
    }
    & > li:nth-child(4) {
      float: right !important;
    }
  }
}

字符串
HTML

<div class="card">
    <p-menubar [model]="items"></p-menubar>
</div>

型
ts

import { Component, OnInit } from '@angular/core';
import { MenuItem } from 'primeng/api';

@Component({
  selector: 'menubar-basic-demo',
  templateUrl: './menubar-basic-demo.html',
})
export class MenubarBasicDemo implements OnInit {
  items: MenuItem[] | undefined;

  ngOnInit() {
    this.items = [
      {
        label: 'First item',
      },
      {
        label: 'Second item',
      },
      {
        icon: 'pi pi-fw pi-power-off',
      },
      {
        label: 'third item',
      },
    ];
  }
}

型
stackblitz