There are many reasons they may have suspected a mainboard, not least of which the fault didn't suggest to them (with their knowledge) that it might be a power issue. Indeed, from what you're saying, it was only at certain times and the rest of the time the UPS performed normally - so it's possible that they wouldn't have noticed an issue with the power anyway.
Honestly, I would have asked the technician why he thought it was the mainboard, because otherwise we're just guessing.
As to availability? Again, no idea - you'd need to ask the technician. Unfortunately, things do go wrong at times - mis-deliveries, out of stock due to unforseen issues and so on. The best starting point is to put the questions directly to the people involved, rather than guessing :-)