"Tender" basically means the boat is tippier than other boats. If true, you'll probably just want to reef a little sooner or be more active in keeping the boat on her feet when the wind picks up.
A buddy of mine had a shoal-draft O'Day 272 that would go over on its ear if you sneezed into the sails. That's tender. The boat was a lot of fun, though. Once its rail was wet, it was quite content to stay there and behaved quite nicely. In fact, he never reefed, preferring instead to just slide a little more sideways. :-)
People have different opinions of what tender is, though, so look at Horner's description as subjective. If you're used to a heavy full-keeler, that Sabre might seem skittish. If you're used to a race boat, it might seem tame. If you're not familiar with either, pay Horner no mind and enjoy sailing her.