Curious what you think they could or should have changed in their supply chain post Brexit, especially since having a base in NI which brings, and I use this term extremely loosely, "benefits".
For EU wide sales (or even beyond?), a logistic hub in the EU - and even a EU trading branch. Benelux area being a popular choice. Triangulation deals still possible, but will admit this is complex and depending on country and dual Tax agreements (which incidentally are constantly maintained bilateral agreements nothing to do with the Withdrawal Agreement) could have been set-up if the sales figures warrantied it.
https://marosavat.com/vat-triangulation-simplification/
It makes little sense to say import to the UK from the EU (say for Mavic wheels manufactured in Romania, then re-sell them to a customer (another importer) in say Portugal - the price simply can't be sustainable). Mutual trading benefit was not on the cards; it was just really silly to believe an existing structure and model could survive. Perhaps the most compelling early example was the collapse of M&S food outlets in the EU.