Have you tested this in all browser types? It seems it is a bug for chrome on Mac OS as the registered formatter command isn't invoked when you press ctrl + b. However, it works on Mozilla and IE. Also, I have found the file and the line number where the command should be invoked from the shortcut key. The file is /libs/cq/ui/rte/core/EditorKernel.js and if you add an OR condition in if the loop to check for the Mac then it does invoke the command chrome on Mac.
However, I would suggest you to raise a ticket in day care for getting this fixed rather than customizing this file. You can just replace <b> with <strong> by means of configuration and get a hot fix for the issue from Adobe.
if (e.isCtrl()) { //Added Mac for chrome browser if (com.ua.isIE || com.ua.isMac) { // handling of Ctrl-keys must be done here for IE var c = String.fromCharCode(e.getCharCode()).toLowerCase(); var cmd = this.keyboardShortcuts[c]; if (cmd) { this.applyCommand(e); return; } else { // prevent formatting shortcuts from being automatically // executed if ((c == 'b') || (c == 'i') || (c == 'u') || (c == 'm')) { e.stopEvent(); return; } } } }