They suggest forwarding all ports in an extremely irresponsible and insecure manner.
Within the port range, enter the starting port and the ending port to forward. For the Nintendo Switch console, this is port 1 through 65535.
So, what ports do I actually need to forward to my Nintendo Switch to play games online? How can I safely and securely play over the internet?
A Reddit thread discussing the same thing did some investigation and found that you don't need to do the entire range. It seems the Switch only needs 45000-65535 UDP:
Now for the Part everyone probably came here to see. The ports, it appears to use a random port within the range of 45000-65535 UDP. I have looked at a number of matches and it seems to always stay within this particular port range so if you are having connection issues try and forward that to the switch
According to that post, they were being a little generous and were rounding down in order to not exclude any used ports:
was rounding the ports out to make sure i got the whole range (example i saw a port used at port 49033 so i went with 45000)
I also found this site which also claims the same range (and also notes how Nintendo says to use the entire range as being dangerous):
Port forwarding works well only when you know the ports your console is allowed to use. In the case of Nintendo Switch ports, you can use anywhere from 45000-65535 UDP.
Lastly, a third site claims the same range:
UDP: 1-655359 or 45000-65535*
where the asterisk stated: "While the Nintendo Switch website states to open up the ports 1-655359. Opening up all the ports could potentially pose a security threat."
