First of all, the way you have set this up you will always see only one text - because your keyframes change the first, second, third Switch nodes from False to True etc., but your 4th Switch is always True. The settings of the previous switches will only show if the 4th is set to False.
More than that, if you want to have 7 text strings: in the current file as it is, there are no keyframes to change the 5th and 6th Switch to True, so of course you will never see any of those other text strings if you do not switch them on. With this setup, choosing strings works like this: set that Switch to True which has the string you want to see in the True input and all other Switches to False (previous switches can be True, but for the sake of an easier overview it would be best to only have the desired one switched to True). If you want to see the very very first Switch node's False input, all have to be set to False.
Apart from all this what I mention above: sometimes it is better to do newer tutorials for current Blender versions. The one you've linked to is for 3.0, we now have 4.2.3 and there have been a lot of changes. Sure, to learn Geometry Nodes in general you can do older tutorials, but some of them do things very complicated which are much easier nowadays.
For example, a while ago Geometry Nodes got new kinds of Switch nodes added: the Menu Switch and the Index Switch. With the Index Switch you can set a specific type of data (in your case String) and change between different entries by changing the Index. This way you don't need multiple Switch nodes that you continuously change from True to False and back, you just animate the Index. The data can be written directly in fields of the node or be connected by inputs.
