My Interrail's summary is inaccurate and probably out of date - it certainly doesn't match the Interrail Conditions of Use, which contain no requirement to produce evidence of your onward/previous travel.
Apart from anything else you may not yet have bought your onward ticket when travelling to the port/border; that's an entirely likely scenario for ferries, where walk-up bookings are usually possible, and plausible for the Eurostar if there's last minute passholder availability. Meanwhile on your "inbound" journey, you may already have disposed of your boarding pass/reservation.
Nor is there a requirement for the "inbound" journey to be the last one made on your pass; if you have a flexi pass you might use it over two or more journeys abroad, so clearly the 2 days of inbound/outbound travel won't be sufficient to cover all such journeys.
I have used at least 15 Interrail passes over the last 5 years or so, and have never once been asked for evidence of my international onward/previous journey - only ever for the ID under which the pass is registered, and even that only occasionally (mostly in the Germanic countries).