In the absence of any data, I'd conjecture that this rose is a product of a breeding line that started with HP 'Frau Karl Druschki' and entailed several generations of seedlings of seedlings being crossed with yellow Pernetianas, HTs, and each other such that the only defined classification that any proportionately major ancestor had was the HP designation of the founder of the line (which I'm theorizing was 'Frau Karl Druschki').

Theory #2, which could be valid simultaneously with Theory #1 above, would be that perhaps the manner in which it blooms and/or grows seemed HP-like (or at least not HT-like) to whoever first classified it.

I don't know if it qualifies as confirmation; but publication Zprávy ("News") of the Czechoslovak Academy of Sciences Botanical Garden at Pru̇honice (1965, p. 23) attributes 'Hold Slunci' to "Blatná 1950," and Blatná was the location of Böhm's nursery.