libinput has a couple of features that 'automagically' work on touchpads such as disable-while-typing and the lid switch triggered disabling of touchpads and disabling the touchpad when an external mouse is plugged in . But not all of these features make sense on all touchpads. For example, an Apple Magic Trackpad doesn't need disable-while-typing because unless you have a creative arrangement of input devices , the touchpad won't be where your palm is likely to hit it. Likewise, a Logitech T650 connected over a unifying receiver shouldn't get disabled when the laptop lid closes.
For this to work, libinput has some code to figure out whether a touchpad is internal or external. Initially we had some code to detect this but eventually moved this to the ID_INPUT_TOUCHPAD_INTEGRATION property now set by udev's hwdb (systemd 231 and later). Having it in the hwdb makes it quite trivial to override locally where the current rules are insufficient (and until the hwdb is fixed, thanks for filing a bug). We still have the fallback code though in case the tag is missing. On a sufficiently modern distribution, udevadm info /sys/class/input/event4 for your touchpad device node should show something like ID_INPUT_TOUCHPAD_INTEGRATION=internal.
So for any feature that libinput adds for touchpads, we only enable it where it makes sense. That's why your external touchpad doesn't trigger disable-while-typing or the lid switch.
 ok, I admit, this is something we should've left to the client, but now we have the feature.
 yes, I'm sure there's at least one person out there that uses the touchpad upside down in front of the keyboard and is now angry that libinput doesn't allow arbitrary rotation of the device combined with configurable dwt. I think of you every night I cry myself to sleep.