My name is***** am another expert on the forum.
I have had a fair amount of experience (and success) with Anti-SLAPP motions. Keep in mind, this motion is a procedural tool, so it is filed in place of a "responsive pleading" as the defendant's first moving paper in a lawsuit, in order to give the defendant an opportunity to dismiss an action early, as well as to have their attorney's fees compensated by the plaintiff for trying to quash free speech.
With the above procedural framework, your matter is still too early for an Anti-SLAPP motion (the HOA's attorney has only sent you a demand letter, you haven't actually been sued).
You can of course respond to the attorney if you wish, your response can be anywhere from "Dear Attorney, my statements are truthful, here are the facts" to "Dear Attorney, My apologies while my statements are truthful I do not want to engage in any form of litigation over this matter and will cease any further communication, but please note I will retain all rights and affirmative claims in the event your client does attempt a lawsuit" (you will need to identify what type of response you wish to draft - if you draft anything, unfortunately this is a strategy consideration, which we cannot advise you of on this forum, but as identified by the earlier expert - do send this via certified mail, and always keep copies of both outgoing and incoming correspondence).
If you are actually sued, then you will want to retain a civil litigation attorney to file your Anti-SLAPP motion. (Remember, to get an attorney's fees award, you must be represented by an attorney, so you have to have counsel, plus, statistically, you have a much better chance of success if you retain counsel than if you were to file "pro per" (on your own)). However, you can file an Anti-SLAPP motion on your own if you choose to do so.
The Anti-SLAPP motion must be filed within the same period of time as your responsive pleading, so you do not have a lot of time from the time that you are served with the complaint, to the time that you must file the motion (30 days from the date of service), or you will lose the opportunity to use this quick remedy (you can still use the same defenses, but you lose all of the other benefits of an Anti-SLAPP motion (quick dismissal, and attorney's fees award)).