What...? No. There are no PCIe packets, it's completely separate. The USB 2.0 wires are not part of the Thunderbolt specification, it's part of the USB C connector specification. USB C is a physical connector with four high speed lanes, a separate pair of wires for USB 2.0, one for negotiating things and one "extra". Here is a TB3 dock with a USB 2.0 port: https://www.bhphotovideo.com/c/product/1512891-REG/belkin_f4...
This is the same reason you see some adapters using DisplayPort alternate mode have a USB 2.0 port only -- in this case, it's DP packets which occupy the four high speed lanes much like Thunderbolt above.
My understanding is that your average thunderbolt 3 equipment, when running in thunderbolt mode, did not directly pass through any USB traffic. Instead a dock with USB 2 or 3 ports had to contain a USB controller that connects back over PCIe. This was very common, lots of docks have them.
"Titan Ridge, however, would disconnect the USB 2.0 and USB 3.1 hubs immediately upon entry into TBT3 mode."
"USB4 (and Thunderbolt 4) don't do this for the classic USB 1.1/2.0 wires of D+ and D-. When a hub is operating in advanced USB4 mode, classic USB 1.1/2.0 signals still ride through a normal USB 2.0 hub"
"disabling PCIe also means disabling the way that all Thunderbolt 3 docks get to USB 1.1/2.0/3.2 devices at all"
This is the same reason you see some adapters using DisplayPort alternate mode have a USB 2.0 port only -- in this case, it's DP packets which occupy the four high speed lanes much like Thunderbolt above.
Yes, Titan Ridge must connect via DisplayPort lanes and you are right, it definitely looks like it only takes five lanes, wow. https://support.lenovo.com/us/en/accessories/pd029622-displa... says two 3840*2160@60hz and a 2560 × 1440@60Hz monitor is supported which is 30.71 Gbit/s -- a third 4K would require 37.62. https://www.dell.com/support/manuals/en-us/dell-wd19tb-dock/... explicitly says it's five lanes. https://www.dell.com/support/manuals/en-us/wd22tb4-dock/dell... even TB4 is?? Wow!