When you get gifted a sub, you can't subscribe by yourself that month, so you lose the chance to get the Founder Badge. So instead, the badge should apply to all types of subscribing.

For example you find a new streamer, you want to sub to it but someone gifts you a sub before having the chance to do it. Later, they get over 10 subscribers so you lose the chance to have the founder badge (even though you were one of the first 10).