I'm wondering how obvious it was that there was a SN session on, and whether the splash park was staffed.
There's a splash park in a public park not too far from us.
It's unmanned, entry is free. There's a ticketing system if it's a peak time, but this has never been in operation when we've been so I don't really know how it works. There are signs by the entrance, but these are small, at (adult) waist height, so an additional sign about a special session would be easy to miss, especially if you're familiar with the splash park and have read the signs before (the signs are mostly about safety advice and opening times).
And advertising on social media, local papers etc about SN sessions - I can imagine families with no SN children skimming over these without stopping to read them properly, as they don't apply to them.
So if this splash park is like our local one, I can easily imagine a splash park SN session being flooded with NT children - not necessarily because they're being selfish or entitled, but because they simply haven't realised there is a SN session on.
Plus, as pp have pointed out, even if it is staffed, there's the issue of distinguishing between SN and NT children, unless there's some sort of pass or ticket for the SN session.