Squarespace is a hosted sitebuilder, not a place that would host a Wordpress site, but it does have provisions for embedding external sources through code blocks and embed blocks.
It’s not clear what SamM specifically means by “spammers would use the forum as a back door to access my site and then crash my site.” Spammers don’t want to crash the site, they just want to flood it with trashy ads and clickbait. If that’s what happened, then maybe Simple Machines lacked Discourse’s tools for controlling spam.
But the question “What sort of firewall exists between the Discourse forum, and my linked website?” seems like something to ask Squarespace. They do warn that injecting code can cause display issues beyond their control, but I don’t imagine an embedded forum post causing more damage than that.