Would Schoon's Hard Shell muskmelon be considered a hybrid as I have read it was a cross between a Bender's Surprise and a Honey Rock.
Goldman surmises that Schoon's Hard Shell is a variant of the Bender, so would have been bred from the Surprise, Irondequoit, and Tip Top, all of which were from the Sill's Hybrid.
In that sense, I suppose it's a hybrid. I'm guessing that you're asking because you want to know if you can save the seed; that is, if Schoon's Hard Shell will breed true or will lose its characters in subsequent generations.
I can't find any specific detail on that, but given that it's sold by Seed Savers Exchange, I'd guess that it is a good candidate for seed saving, as long as you make sure it's not crossing with related species in your garden. Seed to Seed has detailed info on melon seed saving.