I was googling "joplin wysiwyg" and found About the Rich Text editor | Joplin , was happy to find out Joplin has one.
I searched every single option in Tools > Settings, as well as every menu option in the menu toolbar, nothing about WYSIWYG. Opening the Help And Documentation link in the help menu doesn't say either. I started suspecting it was only available in some insider beta version or something.
So, suggestion: the menu should have an option under View, "Switch to Rich Text Editor".
For anyone from the future who finds this thread (I picked keywords in my post title to make it findable), in case Laurent doesn't add a menu option by the time you read this, you activate the WYSIWYG editor by clicking either of the two buttons near the top right of the note you're writing, under the circled "i" icon.