> there's plenty of good reasons to maintain that trade as well
This is true. And in the same breath, how much of Germany's business relies on export to other EU countries and export/import to/from non-EU countries? Germany's leaders and business leaders have already acknowledged multiple times that Brexit will hurt Germany, but nowhere near an extent that means Germany will not protect the integrity of the EU as an economic and political organisation even at the expense of German business with the UK.
More generally UK exports to the EU are about 12-15% of our economy, while exports to the UK from the EU are about 3% (might have been 4%), so even in the EU, Germany's influence in protecting Germany's self-interest with UK trade will only go so far anyway.
This is not a game of pick one anti-EU/pro-Brexit factoid and assume that it is the trump card to all the other issues that all the EU countries are weighing up. The fact is that it doesn't matter how much trade Germany does with the UK - the UK leaving will be damaging to both Germany and the EU but the damage is less important than protecting the EU for the remaining 27.