This seems to be outdated info. Back in the day, we had such restrictions, but since at least 4-5 years it's possible to mix and match Business and Enterprise plans as you see fit. Here's an excerpt from the service description: https://learn.microsoft.com/en-us/office365/servicedescriptions/office-365-platform-service-description/office-365-plan-options#changing-or-mixing-plans
You can combine Enterprise, Business, and standalone plans (for example, Exchange Online Plan 1) within a single account. However, existing limitations on the number of seats per plan do not change. For example, you can have up to 300 seats per plan on both Microsoft 365 Business Basic and Microsoft 365 Business Standard, but an unlimited number of users on Exchange Online Plan 1.