The wheel seems to spin smoothly... I do worry that it will rapidly worsen right before an important brevet.
I have several coming up including a 400k this weekend, a 1000k in a couple of weeks, etc.
The hub was not purchased new, but was serviced by SON only 2500 km ago. These have been dry kilometres... and no powerwashing the bike/wheel, not even from a garden hose.
I've found online old manuals that say that up to 0,5mm of play at the rim is OK.
2,500 km - there's definitely something wrong there and it has to go back. These hubs are normally bulletproof, wet or dry, salt or whatever, you shouldn't have to mollycoddle them.
However, they are right when they say just ride it in the meantime. It's very, very unlikely to simply collapse like bottom bracket bearings sometimes do. Just ride until you have time to send it back to them (though contact them now to let them know, of course).
For reference, I sent my hub back for servicing at around 55,000 km when it had slight play. It's had somewhat over 100,000 km since then and has no play whatsoever. All weather year round riding. There was a couple of years when I did use it only in the winter and swap out to save wear in the summer, but given how long the bearings last I just don't bother any more.