If you can upload a higher resolution picture would be better for diagnosys but seems like leaf septoria, but with this quality picture its hard to know 100% for sure.
If it is leaf septoria whoever gave you the advice not to worry just basically killed your plant as with leaf septoria you should 110% worry and take out all afected leaves imediatly to stop the spread, but if its in 80% of leaves that would be devastating.
I'm hoping i am wrong and its not it. 🙏