There is no proof it's N. ventrata x alata, only people on forums perpetuating that they think this is what it is.
There really isnt all that much N. alata evident in the plant which is common for first generation N. ventricosa hybrids. Consider also that this plant has been around since the early 1980's. N. ventrata was registered in 1979. There isn't enough time in my opinion to cross N. ventrata back to one of it's parents then grow out multiple different clones to find one that would suit the Dutch nursery and produce and market thousands of large plants in hanging baskets by the early 1980s. Deroose still markets to this day as N. alata, which simply denotes that N. alata was the female parent based on their history of keeping things labelled as the female plant. This says nothing about the male parent though. The greatest likelyhood is that it was simply a N. ventricosa as the male parent.