My take (but I'm just guessing, so take it for what it's worth), is that "standalone" is meant to exclude places like Costco that run a gas bar as part of their larger business. Of course you can't use Amex at Costco anyway, so maybe that's moot. Canadian Tire? They say: "
Merchants are typically assigned codes and categorized based on what they sell". I've never used a credit card that gives "4% back on gas, 3% on restaurants, and 2% on space suits for intergalactic travel", but I think that's what they're getting at here. If one of those targeted reward cards would work on gift cards at Shell, then presumably this would too. Most likely you'll be okay.
If anyone
has one of those multi-discount reward cards and can confirm that gift card purchases at Shell qualify as "gas" purchases, I'd really like to know the answer too.