"Available on Mac" having a pretty significant asterisk beside it. The single most critical MS Office Application used is Excel, and even to this day the Mac version is somewhat limited to its Windows counterpart. For very small operations that need little more than an accounting package and a simple no-add-ins spreadsheet, you are right - MS Office and and Quickbooks Online probably works quite well and will run on a Mac.
Once businesses get into the "M" in "SMB" area, Mac Office really starts to show its limitations. Many of the required add-ins to do certain types of analyses (such as
@risk for Monte Carlo simulations) are not available for the Mac version of Excel. Other Office applications such as MS Project or Visio are not available for Mac at all (unless you want to use the Web versions of those apps, which are VERY feature-limited). Tools that tie into ERP and CRM systems for certain types of reporting (particularly report development, such as Solver designer, SSRS Report Builder, PowerBI Desktop). Businesses of this size also tend to outgrow their accounting software, so QBO is no longer feasible.
Again, with a huge asterisk in some cases. Yes, most web-based applications will work well on a Mac or on a PC or on a Linux box (heck, they'll even run on Haiku or ArcaOS nowadays). However, there are often supporting applications that work with those systems that are either Windows only or rely on Windows networking technologies to provide a fully supported experience.
I work with one such system (which also happens to be one of the fastest growing SMB ERPs in the world today) - Dynamics 365 Business Central. Anyone can access and use 365BC from any supported web browser, including on a Mac or Linux system. Accessing any part of the ERP works the same for any user using a supported browser (Safari, Edge, Chrome, Firefox, Omni - they all work quite well as far as I've seen). HOWEVER, there are some capabilities that require Windows (some are trivial, some not so much):
1) Active Directory SSO. Not a big deal, you just have to re-authenticate when you log in from a non-Domain-joined computer.
2) Open in Excel: This one is actually pretty huge. Most areas of the ERP that have bulk-updatable areas (think Customers, Vendors, Items, Transactions, Journals) have the ability to be opened as a table in Excel for mass updating or for ad-hoc reporting or data integrations. This only works with Excel in Windows because it is not just a simple spreadsheet with data - it is a fully-authenticated interactive session between Excel and the ERP system.
3) Modifying external report layouts: This is more an administrative tool, but still only works on Windows machines - most reports (think invoice layouts, purchase order layouts, etc - that are transmitted as .PDFs or printed) are maintained using a tool called Report Builder.