Nonsense, it was published in 1991, it isn't in any sense an early RPG. For instance you have fairly obvious games like Space 1889, which was published several years earlier and surely qualifies as a mixture of Fantasy and Science Fiction. Beyond that there are many other similar examples, going all the way back to Gamma World. I would point to much of the work of Kevin Siembieda (such as RIFTS, published in 1990) as also being in a very similar vein. I guess you could argue that 'platform systems' like GURPS don't directly and explicitly MIX genres, but they provide rules for many genres, and thus a commonality of rules platform in which such mixed genre play is easily facilitated.
I mean, in the early 1980s our 'D&D' campaign with one particular GM consisted of a mixture of 1e AD&D (with bits of B/X mixed in), Fight in the Skies, Boot Hill, Gamma World (the original one), Traveller, and various other RPGs simply mixed together in a huge mishmash of play. AD&D 1e DMG even provides rules for conversions of characters between D&D, Gamma World, and Boot Hill intended to allow them all to appear (with their various game subsystems) in the same adventure. If that isn't cross-genre play, what is? My understanding is that Blackmoor itself was a mashup of several different genres without any real sense of it being purely one or another.
So, no, Synnibarr is not 'breaking new ground' here. Not in any sense, not all the way out here in '91. If it had appeared 10 years earlier, the claim might be somewhat more credible, though I would still assert that there are pre-1980 RPGs which might even still beat it (but at least it would be arguable).