You are correct that a wheelchair ALONE won't get you into the alternate entrance. A lot of the queues are/were wheelchair accesible. But what started happening was people would rent a wheelchair AND come to Guest Relations for a "Wheelchair" stamped GAC (this stamp was not intended as an "immediate entrance" tool).
Eventually, the parks (especially MK) replaced the "Wheelchair" stamp with the "Alternate Entrance" stamp. This meant that simply by having a wheelchair, your GAC was stamped for "Alternate Entrance," meaning FP line usually (or a side-door).
I mentioned a week or so ago that Epcot seems to be the ones having the most issues with switching to DAS. They were also the park that was least likely to offer a Guest in a wheelchair the Alternate Entrance stamp. They would do everything they could to only stamp Wheelchair. MK didn't even have the wheelchair stamp. Why the inconsistency? Talk to the park VPs. Anyways, what this led to was MK Cast Members not knowing what the "wheelchair" stamp was (which would have made the Guest wait in the standard queue, usually) and sending the Guest to City Hall to get Alternate Entrance (which would send them through FP).
This info must just get around, because Guests with wheelchairs usually knew to ask specifically for the "card with the arrows (Alternate)." Whether it was internet, or word of mouth, or both, I don't know.
So while you are correct that initially wheelchairs weren't supposed to get "front of line" access, in the end it turned out that they did, in general.
But you didn't need a wheelchair to get a GAC with an alternate entrances stamp. That step was pointless and cost the fools extra money. I wish they'd created a stamp for "must use wheelchair ride vehicle". That would have been a lot more helpful in weeding out the abusers while helping the needy. Few cheaters would race to get a GAC if they knew it meant they'd then have to wait in another line at the alternate entrance for the one vehicle to take them around the track.
And while we're at it, why can't the park buses be designed like the big motor coach buses? Then the ECV users would park their scooters under the bus in storage and the wheelchair-bound users alone would be loaded onto the bus taking up those precious seats. (Yeah I know, unintended consequence is that it takes so bloody long to work one of those bus wheelchair lifts and they are so prone to breaking. Ramps are much better because they have a manual override. You don't get stuck on a bus with a ramp; but if the lift breaks you have to wait for Maintenance to carry you off.)
I know, let's have an ECV towing trailer at the back of every bus. Then those riders can park and board like everyone else saving the wheelchair spots for people who actually ride in the space.