Those tachos are really good 8)
I *did* use a screened cable; the same kind of stuff intended for an electric guitar, so I can't comment on performance with un-screened cable.
As you say, there is very little current flowing, which means you could use as thin a cable as you like; it only needs to be mechanically robust.
However, because there is such little current flowing, an un-screened cable will be susceptible to electrical noise interference which *could* cause the tacho to mis-read.
That would depend on how long is the wire run (it is pretty long) and how close you run it to other wires that are carrying, and importantly switching, a lot of current, or what other electric noise there is in your environment.
Anything like the windscreen wiper motor, or other nearby electrical circuits (even from other cars) *could* couple noise into the tacho without proper screening.
I used something like
https://www.ebay.co.uk/itm/GOLD-6-3...1m-2m-3m-5m-10m/351785922954?var=620702262465; I cut off the jack plugs and soldered on the appropriate connectors, ignition-signal spade and a ground wire at the coil end and (IIRC) signal wire straight into the tacho connector at the front. With heat-shrink to keep all the joints strong, neat and insulated.
I don't know how much arse-covering there is in the manufacturer instructions.
The gauge seems reasonably well damped, so maybe any electrical noise wouldn't be noticeable at the needle anyway.
