I don't know her, but in general I find it's best to start under the assumption that people aren't lying, and generally they are trying to communicate something they feel is important.
However, I also recognize that people see things differently, and may be having a harder day than normal, so it's usually best to say, "This concerns me, since this is different than what I've been hearing up until today. I don't have time now, but I'd like to sit down with you and at least one other of the daycare staff to discuss how long these issues have been occurring and their severity."
Then set up a time - hopefully within a day or two - to do just that.
Make sure you hear it from more than one person, so it's a bit more objective, and ask for specific examples of the poor behavior, how long and how frequent, is it constant, or does it occur in bursts throughout the day. Then ask what they do to redirect and teach him, and what they do to avoid these issues. Does someone check on him every few minutes to make sure he's doing ok, or is he left to his own devices for long stretches of time? Are his interactions with other kids normal, or is he getting frustrated because others are not sharing, or forcing him to share? How large are the group activities that he is showing disinterest in? Is it all group activities, or only certain ones?
The intention is to meet together at a time when everyone is happy and awake, rather than at the end of the day and tired, and understand all the issues, and the best course of action. Making sure they understand that you are pursuing diagnostics that might lend some insight. Also, make sure they understand that you are interested in discussing these issues with your son and teaching him at home as well so that things go easier for them - this is a group of people all interested in his well-being, and all are willing and able to work towards that common goal - you don't expect them to do everything, and they shouldn't expect you to do everything.
You should also make it clear that while they themselves might use ambiguous and outdated terminology such as "slow", they should refrain from using it around you and your children. If they must refer to disorders they should use medical terms so that you can both understand what is being talked about. The term "slow" may be offensive to some, but worse - it's very ambiguous, and doesn't lead anyone to try to understand the problem, it only leads to further misunderstanding. Further, unless they have the ability to perform a medical diagnosis, they should only ever discuss symptoms, and allow you to discuss them with your medical care provider for further diagnosis.
In general, I don't think people who talk like that mean harm, but they should be gently reminded that you have boundaries and they crossed it, but you do want to resolve the issues with their help.