You got me curious enough to check it out. Ad or not, that is one of most incredible sites I've seen with deep resources that are very valuable. I learned a lot in the 20 minutes there. The 'interstitial' is just that -- a plea for donations to support the writer and hosting for an ad-free (it's ad-free for everyone, not just donors), high-quality resource. I had to read the page 3 times before I picked up on the instruction to input the root URL into the address bar, instead of being linked there. Haven't looked at the code, but is he just looking at referrers and dynamically routing based on being on his "graylist"?
If I find myself there more, I'd be inclined to send him something.
As to its effects, I think he'd prefer to drive off any and every reader who is offended by the ask. They are just costing him bandwidth with zero chance of any return. Maybe they share it with someone who ends up donating, but people who actually donate are much more inclined to be good sources of referrals to other people who may donate.
As to whether it equates to posting a link to an ad or promotion, I really don't think it crosses the line. This isn't one of those informercial sites that lead you on for an eternity without ever providing anything of substance. Instead this is just an interstitial on steroids meant to drive off undesirable traffic. It's 1000 times better than annoying ads and/or anti-blocking measures, imo.