I do not think that white has any advantage in the Reti, especially after 2...d4.
AlphaZero taught us that space is very important, probably even more important than we previously thought, and with 2...d4, black gains space. Stockfish 11 says that black has a -.38 advantage at a depth of 32, which needs to be taken with a grain of salt as it probably will dissipate as they get further into the opening, but it probably does indicate that black is absolutely equal.
In addition, in practical play, according to the Mega 2020 database, after 2...d4, white has a miserable winning percentage of only 46.9%.
That last statistic probably indicates that Horowitz was correct in that it is not the best opening (it is also not horrible), but you also need to keep in mind that unless you play very strong masters, you are probably fine to play it, although if you think you might get to that level in the future, then you also do not want to have to rework your openings totally when you get there.