I suspect "64bit" here might not refer to CPU chip sets and software operating systems, which has been the commonplace meaning for the past several years.
I'm guessing this might actually be very good news for users of Reaper + Synchron libraries, but we need to hear from a VSL dev to tell us for sure.
If I recall correctly from the couple of years I spent with Reaper, this DAW has long had the superb advantage of a 64bit-clean audio path all through its instrument channels, VST inserts and summing nodes. Wherever a VST instrument or VST Fx insert cannot accommodate 64bit audio, then without any fuss or bother that channel's local audio path will be limited to whatever resolution the instrument or insert offers - usually at least 24bit (which has been a standard audio resolution in DAWs pretty much since the earliest days).
I could be wrong, but "64-bit bridged" might perhaps mean that Synchron Player sends its audio out in Reaper's 64bit format, such that the following local audio processing can take advantage of VST Fx plugins that operate at 64bit audio resolution (if available); and/or at the very least, Synchron Player's audio will be presented to subsequent summing nodes (in Reaper's mixer) as 64bit so that 64bit summing into the mix takes place for that audio channel.
I can tell you that the most exquisitely detailed drums mixes I've ever heard were when using Superior Drummer 3's core library in Reaper, even though SD3's audio samples are all originally 16bit, the same as in Synchron libraries. Reaper's 64bit-clean audio paths and summing are a very splendid thing.
Is my guess here just wishful thinking, VSL?