Horwitz has hit at every level and has a MLB OPS+ on par with Reynolds. I'm not seeing what's "seriously optimistic" about assuming that he'll be able to provide sufficient hitting at 1B going forward. He doesn't have ideal power you want out of a 1B, but he has a career .307/.413/.471 slash line in the minors and a .264/.355/.428 slash line in the majors over 2400 career PAs and averaged a HR every 40 ABs in the minors. For comparison, Reynolds' minor league slash line was .312/.373/.472 over 1200 PAs and he averaged a HR every 39 ABs in the minors.
I'm obviously not going to equate Horwitz as the next Reynolds because Reynolds had a huge power surge going from the minors to the majors, he went from a HR every 39 ABs in the minors to a HR every 24 ABs in the majors. There are also things that Reynolds has/had that Horwitz doesn't have, namely good defense in the OF (in the past) and good speed. That said, I don't see anything statistically that makes me doubt that Horwitz should be a good or better MLB hitter going forward. He has the good minor league numbers, good major league numbers and good batted ball data in the majors (60th percentile xBA, 74th percentile xwOBA and 61st percentile xSLG). He just doesn't have the ideal HR power you want out of a 1B, his hitter profile is that of a good hitting LF or 2B.
I said this earlier in the off-season, but if Horwitz was a solid defensively LFer and they signed Walker to play 1B, I'd be over the moon happy with this off-season.