Visitors' IP addresses are provided to Simple Analytics in the course of loading their script and reporting back the results. That's all it took to get web sites using public Google Fonts resources in trouble—note that this didn't involve any actual analytics scripts or overt data collection, just some embedded CSS and font resources.
The only real advantage Simple Analytics has here is that they aren't Google, so they aren't as much of a political target and don't have deep pockets to attract legal predators on the lookout for an oversize payout—which is a pretty thin justification for treating them any differently.
The regional Google Fonts ruling was an odd one. It had to do with Google processing the IP address, not whether the website was loading from any external domain at all. It did appear to be based on the court's misunderstanding of an IP address contacting a server to be data processing, and perhaps we're going in that direction, and won't be able to use even an extremely privacy-focused CDN without a formal data processing agreement, but that is not currently the intent of GDPR.
The advantage of a service like Simple Analytics remains; it does not store or process any user data.
The only real advantage Simple Analytics has here is that they aren't Google, so they aren't as much of a political target and don't have deep pockets to attract legal predators on the lookout for an oversize payout—which is a pretty thin justification for treating them any differently.