Yes, it is significant. That much change so quickly could cause a lot of damage in only 24 hours.
I know you don’t have answers you want... but in our PMs, I previously sent you a solution that will do exactly that. To repeat for people here, test Alk (and calc) more frequently, and do at least 2 tests back to back each time. This will give you insight into the validity of your results, as well as potential causes/issues. So do it. Then let us know the results. Follow the rest of the advice too. It’ll help. The “what-ifs” of your results will provide answers to numerous questions.
As I’ve also stated, don’t make any big changes. Only small changes on occasion, and frequently test your water when doing so. Stopping something cold-Turkey that you do regularly for months is a huge change. Don’t do it blindly. Honestly, you have no way to know how helpful ATI been. But for example, if you stop and then everything dies in 24 hours, then you will have learned the hard way... or... you could just do 2 tests, a few times a week, for 2-3 weeks, learn more about your tank by getting the answers you want, and then make an educated decision.