I don't think it's as much of an attempt to buff engi, as much as reduce the effectiveness of Demo/Soldier. The two are some of the most powerful pub classes, and tons of noncomp players resent that. This entire update kind of felt like a bunch of indirect nerfs or paradigm shifts for soldier. Gunboats are less effective in pubs because you can get neutered by the SC, shotgun is a lot more attractive because flying scouts, and the buffs to the banners make them look a lot more attractive for players who want to be supportive. If their goal is to make certain classes more effective for full time running, which fits their whole HL positive agenda, this buff is a solid attempt.
The SC itself isn't that powerful as an assault tool, so it's hard to pass judgement on it before it's used in some HL matches. Wrangler is still offensively powerful, and raw tanking + better DPS might still be better than hard denial to projectiles, especially with classes like heavy and spy being fairly common occurrences in HL. Valve is apparently terrified of changing core mechanics of classes 7 years in, so they have to band aid unlocks in an attempt to make classes that are otherwise ineffective (pyro) or so skill less as to be out classes by other classes in high level games (heavy, engi). The actual goal isn't bad, but I'm fairly sure valve don't know enough about HL/6s to actually approach it correctly