The question remains, how can these statements:
".......because the sealed runs out of excursion at much lower levels than ported."
"Ported cabs limit driver excursion more than sealed....."
both be true? A sealed cabinet must allow for more, or less, excursion than a ported cab. Can't be both.....unless it's a quantum physics thing?
The first sentence is to be read as "the sealed runs out of
clean excursion at lower (signal) levels than ported", because at that point (again, lower than ported) the driver is not impeded by the enclosed air mass from reaching its overexcursion point (where this point is in millimeters is inherent in its construction but, as said, when it is reached relative to input signal/volume level is dependent on cabinet size and ported vs. sealed - and in the latter case specific tuning frequency - configuration), past which distortion and ultimately mechanical damage occur.
In light of the above, "[p]orted cabs limit driver excursion more than sealed....." is another way of saying the same.
Or, to be clearer: "excursion" is either of two things:
1) in sentences such as "this woofer has plenty of excursion" it is the inherent property of a driver to move back and forth without distorting first and damaging itself later. The threshold is determined by construction details but the driver staying within that [safe (clean)] area is related to the box and tuning it is installed in, and input signal - give 'er enough watts and it will over-excur, unless it fries first;
2) simply, the to and fro movement of a driver. That an alignment (ported/sealed) "limits excursion" means simply that the driver's movement is opposed by the air mass behind it. For bass applications, usually, it is a good thing that the movement is kept in check within the clean, undistorted zone that the driver has been designed with (but which, again, is not enough to prevent a clean signal and safe operation by itself: use of a driver in free air is only safe with small signals, past which it needs crucial help from its placement within a cabinet, as said); for all applications, not just bass guitar, it is self-explanatorily important that the driver's movement is kept at least inside the (wider) safe, breakage-free zone.
As for sealed vs. ported, the latter, somewhat counterintuitively (given that it's open to the outside), provides a "cushion" to the movement of the driver because of the air being excited to resonate at and around a frequency, adding to the output of the driver in the same bandwidth in the process (which is its main claim to fame), but fails to do the same at some point below the resonant frequency, at which point the driver is "unloaded" and flaps about. In a cleverly-designed system this happens only in potency, because the sub-bass dangerous zone is below what the signals actually contains. A sealed cabinet is more gradual in its letting go of the driver and not stopping it from moving too much, but all things being equal, it is less efficient at the job in the relevant roll-off zone of the response.
tl;dr version: there is no contradiction. In both sentences there's a moving thing (D) mounted on the side of two boxes, (S) and (P).
In sentence 2), S is better than P at confining the movement of D.
In sentence 1), P gives in before S trying to confine said movement, so D...runs out of available (healthy) movement.
