::: I need to expand this a good bit further. Sipe senior invented a process for siping, but he didn't do it for anything closely related to the process as understood today (the problems of early solid tyres are so crude that this just wasn't an issue yet). The invention of deliberate siping (or "knife-cutting", in Europe) as a deliberate measure to improve wet performance appears to be Belgian, from the Engelbert company in 1935. It was picked up shortly afterwards by Michelin, who really popularised it.  The first commercial use of siping was a year earlier, where Tecalemit used a knife-cutting process almost identical to Sipe's original patent as a means of recutting worn treads.  Europe also abandoned the slow process of literally knife cutting in favour of steel blades in the tread moulds, forming the sipes by moulding.  Sipe junior was a long way behind Europe, his contribution was to popularise these water-clearing blades in the USA where they'd so far been ignored. 
 ::: As to the Sipe senior stories, I don't believe a word (but can't source this). The widely spaced sipes in a solid tyre would have negligible water-clearing effect. It's not the slits that clear the water, it's the thin rubber blades the tread is cut into. Narrow slits are only needed because the blades need to be both narrow, and still supported by adjacent rubber, so a blade moulded with wide gaps wouldn't last.