I really cringe when I see a new tutorial that uses any 1.x binding but most especially the MQTT 1.x binding. Essentially this tutorial is a dead end as MQTT 1.x is not supported on OH 3 at all. We’ve been working really hard on this forum to push people to the MQTT 2.x binding for at least two years now.
We really do appreciate the time and effort put into this tutorial and are glad you posted it. But in the long run it’s going to be a problem for us and many OH users as it’s impossible to use with OH 3.
For how to configure an MQTT Event Bus in the MQTT 2/3 binding see MQTT 2.5 Event Bus. This should actually make your Item configurations easier because you won’t need to apply any binding configuration to your subscriber Items. It is also “future proof” in that users of OH 3+ can actually implement this.
Or, if you are on OH 3, you can skip the MQTT entirely and use the Remote openHAB Binding.