Yea the Seneca reservation license is for the NY part of the reservoir. My understanding is that pretty much the entire NY side is a Seneca reservation so that’s why you need the Seneca license rather than a NY state license.
I tried to find the Seneca reservation license online somewhere, but it doesn’t look like they are available online. You can only buy them in person (and when I bought them we had to pay cash).
I don’t know anything about silver creek specifically but I’d guess you would also need the reservation license to fish whatever water is within the boundaries of the reservation. I’d double check the regs just to be safe though.