JRE's answer has already identified that the connector on the Nordic board is a switch type, so none of these examples have both antenna options connected at once.
As to the other part of your question - why can't both be connected at once - the answer is impedance matching.
The integrated antenna will be designed to present a particular characteristic ...
I took a look around the Nordic documentation.
You don't need a jumper on the Nordic board because the RF connector it uses has a switch.
The connector is of SWF type (Murata part no. MM8130–2600) with an internal switch. By default, when there is no cable attached, the RF signal is routed to the onboard PCB trace antenna.
I don't design these kinds of antennas and my books on the topic are at work, so I'm not sure, but you will find you already got into trouble with your length calculation. You used the speed of light in a vacuum to calculate the length. (notice the 'in a vacuum' part). On a PCB the wave will travel more slowly. Your microstrip tool helpfully points out that ...