I think you are both right in your own way. McGinty was a horrible candidate for the Dems - if they had a better candidate, Toomey might have lost. Toomey played to the country-club pubbies in the Philly burbs who otherwise disliked Trump, and Trump played to the Reagan Dems in the rest of the state who dislike country-club pubbies. There are enough of both to run an inside straight and win.
Keep in mind that there wasn't a single major poll in the last few weeks that showed Toomey winning this race. Most of them showed McGinty ahead, or listed the race as a toss-up.
As recently as six months ago it was almost universally accepted that the Democrats would win control of the U.S. Senate in this election cycle -- simply because 24 of the 34 seats that were contested this year were held by Republicans. The Democrats would have needed to pick up 4-5 Republican seats, depending on how the presidential race ended up. Toomey's seat would surely have been one of the ones that ended up in the Democrat column if this scenario came to fruition.