Ok I figured out a work-around for this!
Within Settings > Main Settings, I checked the box: Disable Login Notices? YES. Turn off the notice strongly suggesting volunteers login before signing up for a volunteer slot (on signup form page) and the notice to login to view/edit signups on the main volunteer list page.
I then added a text block under my sign-up sheet short code with the custom text I wanted instead.
There are filter hooks for all the front-end text output by this plugin. So, with a tiny bit of programming you can modify any of the text. Or, if you don’t want to figure out the code, my cheap Customizer extension lets you change all the public side text, plus it has many other features for customizing the way things appear on the public side.
https://stephensherrardplugins.com/plugins/pta-volunteer-sign-up-sheets-customizer/