That's the old heatmap, pre-update.
Also, servers run specific configs that influence gameplay. Most 24/7 Dustbowl/Goldrush servers (Which will get the majority of hours played) allow up to 8 engineers. It's hard enough assaulting 2 sentries and fighting the knock back. You get a nice setup and a lot of area's are unbeatable. This just happens to be one (and the first) of many chokepoints that can be locked down in these maps by unblanaced class setups.
Valve have noted this difficult pass and have already modified it twice so saying Valve havn't done anything in regards to heatmap data is just incorrect. It's even posted in the TF2 blog. Secondly, the first stage is going to be played 3 times more than the final stage, and twice as much as the second, so again, these stats are also warped in this sense. If heatmaps were able to compensate for this variable, you'd see it is a lot more balanced than it appears.
This chokepoint could be more balanced for the average dolt that plays TF2, but anyone with half a mind doesn't just run through that pass when it's covered by sentries, snipers, ambushing pyro's and spy's and demo and soldier spam. The key is to gain access to upstairs, which can be done by bypassing this chokepoint. Having just 3 or 4 people on your team that realise this will get you to the next stage. The gameplay flaw on this map is not that this chokepoint is over powered, but that players that flank the defenders are more inclined to attempt to spawn camp and fail than to retreat to the objective and push the cart because the enemy spawn is closer. This is why people don't get past the first stage.