On one hand - it's close that it probably infringes on the any patents that cover React
On the other - it's not infringing patents that scares people - it's the patent clause in the React licence. If you're not using React, you're not accepting this licence.
So Preact seems like it would safe enough - unless you believe that Facebook would one day go on the offensive with React-specific patents. (for the record - this seems unlikely to me)
If Facebook goes on the offensive, you are protected if you use React but not if you use Preact.
If you sue Facebook for patent infringement and they use their patents defensively, you are in exactly the same situation using Preact as you are using React; you don't have a license for the patents in either case.