You say this like there isn't a precedent for special sets not always making it through the localization process intact. Super special sets like 20th Anniversary Expansion Pack (XY Generations) and 25th Anniversary Collection (Celebrations) made it through OK, but every now and again sets like Champion Road and GX Ultra Shiny have been chopped up and remixed so much as to be unrecognizable in the localization process.
I think the signs have been around for a while that the English localization 151 would be a near-straight port – the way it's structured, it would've been impossible to mess with it too much or the entire set numbering gimmick would've been lost – but again, when you look at the way a set with a strong, cohesive theme like Champion Road was willingly scuttled by TPCI, there was no guarantee that 151 would get to remain intact until we saw those signs.