They claim that every aspect of your holiday is covered. They are ABTA bonded for complete packages but if you only book one aspect then this seems to indicate that they have insurance to cover this, although like most things, I guess its open to interpretation.
http://www.alditravel.co.uk/faqs.php#sfc
They do have a fair amount of weight behind them, they own over 5000 supermarkets worldwide and have always been in profit, at the end of the day we all need to eat!!
Nothing is guaranteed in this life but I'd feel just as comfortable booking with Aldi as anyone else.
Mark