If you think about a king and his castle; people may come to take it and all he has to do is repel them in order to be the king. That’s what the belt represents. So I say yes, it’s a successful title defense.
The “title” is just a belt around one’s waist. At the end of the day getting the Win is what matters most and neither fighter got the win. The fight was a waste of everyone’s time since there was no true victor. Both fighters wasted months of time training for this fight and the fans who paid money to be there wasted their time and money
Dustin Poirier, despite never being an Undisputed belt holder, ranks higher than Conrat McTapper on the all time list. That’s because his resume of wins is far better than Conman’s is, despite conrat holding two belts
According to Grasso’s wiki, not an official source but concurs with the above where it would normally say “defended” but it only says “retained.” Alexa Grasso - Wikipedia