5k @ 60Hz needs either two DisplayPort 1.2a cables (8 lanes) in 'MST' mode (where each cable drives half of the screen) ...or a single DisplayPort 1.4 cable (4 lanes - faster speed), but in the latter case the computer's GPU, monitor and the USB-C/TB controller all need to support DisplayPort 1.4 and single-stream 5k.
Until last year, Thunderbolt controllers didn't support DisplayPort 1.4 at all, but since Thunderbolt has a higher bandwidth and multiplexes DisplayPort and PCIe data into a single stream that can use all of the wires in a USB-C cable, it could fit two 'virtual' DisplayPort 1.2 streams on a single cable and drive a 5k display in MST mode.
USB-C alt mode (i.e. a non-TB display and/or a non-TB computer connected via USB) simply works by 'switching' some of its 4 high-speed data wires to carry raw DP signals, and has always supported DisplayPort 1.4 on paper, but that doesn't mean that all USB-C controllers support it - none of teh early implementations did, the Intel controllers in pre-2018 Macs don't, and its irrelevant to the 13" MBP and Air because their Intel iGPUs don't.
Maybe the iPad Pro can output 5k DisplayPort 1.4 over USB-C, maybe it can't, but the odds are that the LG display only implements it via 2xDP1.2-over-Thunderbolt, because most of the Macs out there can't do DP.1.4.
(Presumably Apple's new $1k XDR display stand can use DP1.4, at least over Thunderbolt, but you need the optional $5k display for that).
The previous version of the 4k was USB-C DisplayPort Alt Mode only, and would work with an iPad or 12: MB, while the previous 5k was Thunderbolt only and would only work with a MBP or Air. Pre-2018, unlike the 'host' ports on computers, the socket on a peripheral device could only be one or the other - but the latest Thunderbolt controllers fixed that.
Both of the revised displays now have dual mode inputs that can work in either Thunderbolt or USB-C Alt Mode.