Thickness and density matter more than pressure. As does the velocity and mass of your impactor.
The portion of Venus's atmosphere that has a pressure of > 1 bar is approximately 50km thick. For the sake of argument, let's say that the entire 50km column is pressurized at 92 bar (which isn't the case; you only experience 92 bar at/near surface level and then the pressure gradually declines with increasing altitude, as it does on Earth).
Now what we want to know is the density of that atmosphere. On Earth at STP, air has a density of 1.2754 kg/m3. The atmosphere on Venus, however, is predominately CO2, which is a bit more dense; approximately 2 kg/m3. Scale that up 92 times, and you've got 184 kg/m3 (note that this is actually about 3x the value reported here, so I'm being quite generous, probably because I've not taken Venus's extreme temperatures into account). For comparison, water has a density of 1,000 kg/m3 and rock can be up around 3,000 kg/m3. So while the atmosphere on Venus is quite dense in atmospheric terms, it's still not very dense at all in terms of 'things a meteorite might encounter'.
But the question is, will interfacing with that atmosphere make an appreciable effect on an impactor the size of Mars? We can use some math to work out how much of the impactor's surface encounters the atmosphere before it reaches the surface of Venus. This is on the order of 100,000 square kilometers (rounding up to give the atmosphere the best possible chance of affecting the outcome). Let's say that all 50km of atmosphere within that entire area must be moved out of the way for the impact to occur (not actually the case, but again we want to give the atmosphere the best possible chance). So that's 5 million cubic kilometers of atmosphere that needs to be pushed out of the way, with a mass of 9.2 × 10^17 kg.
So how massive is Mars/Theia? Approximately 6.39 × 10^23 kg. Or in other words orders of magnitude more massive than the atmosphere that it needs to push out of the way. And how fast is the relative impact speed? 50km/second is within the ballpark, and works nicely with our 50km thick atmosphere.
Anyways, for an impact to happen we have to move 9.2 × 10^17 kg of atmosphere 177km (assuming the worst possible case mathematically, with the entirety of the atmosphere sitting right at the point of impact) in 1 second. That takes about 5.765 × 10^28 joules away from our impactor's total kinetic energy.
Problem is, at a 50km/sec approach speed our impactor has 7.988 × 10^32 joules of kinetic energy. Which is still orders of magnitude more than it will expend plowing through that atmosphere. Even if we drop the impact speed to 10 km/sec, we're still talking about 3.195 × 10^31 joules.
In other words, even in an unrealistic best-case scenario, Venus's atmosphere is basically transparent to an object the size of Mars. It doesn't prevent the impact or even appreciably decelerate the incoming projectile immediately before impact. At best it reduces the final impact energy by ~0.2%. Which is more than I thought it would do when I started this exercise, but still not nearly enough to save Venus from disaster.
The shockwave that gets created when that dense atmosphere is brutally shoved out of the way would probably be incredibly destructive by Earth standards, however.