The text alignment buttons work because you know you can't have both centered and left, otherwise relying on subtle spacing would mostly signal some connection, not mutual exclusivity (similar to how those | symbols group related buttons together)
One solution to selectable labels would be to add the indeed missing indicator, e.g., you could have the whole label have a border when it's active (though in this case maybe you don't need radio buttons, but just have multiple lines that look like distinct lines?)
Hard to access the background group, I know it doesn't work if it includes labels like in the screenshot in another comment, but maybe only for buttons it'd be more understandable
I thought there was an actual difference in presentation back in the days toolbar buttons actually had borders, but no, when you look at https://winworldpc.com/res/img/screenshots/6x-a08a659bdbb00f..., Word 6, you see it’s all just simple grouping of toolbar buttons, nothing to do with mutual exclusivity at all: New|Open|Save, Print|Preview|Spellcheck, Cut|Copy|Paste|PasteFormatting (if my memory serves me correctly?), …, AlignLeft|AlignCentre|AlignRight|Justify, Numbered|Bulleted|Dedent|Indent.
I thought it was true when compared alignment group with bold/italics, in the Word Ribbon UI they do seem to have less space separating them, but then in Wordpad the paragraph button next to alignment group seems to have the visually very similar space width and the icon is very similar, so there is no way such poorly perceptible convention helps even if it does exist
One solution to selectable labels would be to add the indeed missing indicator, e.g., you could have the whole label have a border when it's active (though in this case maybe you don't need radio buttons, but just have multiple lines that look like distinct lines?)
Hard to access the background group, I know it doesn't work if it includes labels like in the screenshot in another comment, but maybe only for buttons it'd be more understandable