a man who wanted to get out of something might write a letter that is not entirely truthful
How came he to imagine that? Badly thought!
Just because his anticipated pleasure is not to take place he sinks to think meanly of Brandon's motives. And he encourages Marianne to join him although he - being several years her senior - ought to know better.
It's as if he brings out the worst in her.
Somewhat like Emma tricked into being rude at Box Hill - by Frank Churchill.