This isn’t true for most Unreal Marketplace users.
Since the cutoff point for commercial activity where you need to switch to Professional license is at 100k USD / year most people will be just fine.
IF you’re making more than 100k / year already… the cost of marketplace purchases are likely a tiny fraction of your total company expenses. 
The only people I see who could get in trouble with this is who make like 105k USD a year, so they are just over the limit. However this is true for waaay more expensive things like software subscriptions etc. where you suddenly need business licensing etc.
So FAB will likely be one of your smaller problems there. Still, I wish they went with 120k limit like many other software companies have, I believe it’s more realistic these days.
Source: https://dev.epicgames.com/documentation/en-us/fab/publisher-get-started-in-fab