I don't think you need more than a few inches clearance for a rear-ported speaker. Certainly not more than a foot.
I call BS. I'll make some leeway for small bookshelf speakers with 1" ports, but, absolutely not for towers or subwoofers.
If the port does not have enough room and air flow is congested, it is no different than intentionally plugging the port with foam plugs. The latter is a known way to change tuning characteristics of a bass enclosure and used in many towers and subwoofers.
The port also adds a high Q nature bandwidth peak at the tuning frequency. This coupled with room gain from extremely close placement to room boundary will create a bloated low end.
The two effects combined will wreck mid to low end FR. Perhaps, you've serendipitously arrived at your observation, but, the physics guides otherwise.