I would agree that the keyboard doesn't make a very big difference in speed...For the most part a given person will type very close to the same speed on any given keyboard. There might be some changes for each individual given their preferences, style etc....but given a reasonable time to adjust, most people will probably find each keyboard to be very similar result wise or within a relatively tight range.
BUT, some people will just enjoy using other switches/keyboards more than others. Their enjoyment of using the keyboard will be greatly enhanced when using something they like more than others..and even if it is possible they type faster on another type of switch, enjoyment is huge.
There are tons of users that will type slightly faster on say, browns than they do on their favorite reds..but they don't like using browns..they like reds. So that extra few WPM is meaningless because frankly, maxing out your WPM is not a major factor for most people...Or why get Topre over a normal Rubber dome or even Cherry MX over RD if you type the same speed? It is simply because you enjoy using them more..they feel better...the experience is enhanced...It isn't an output or objectively 'better' keyboard..it is purely the enjoyment of using it.