1. Only small percentage of people was afflicted with physical symptoms.
2. These symptoms were vague and so variable that they had to refer to as "syndrome", rather than "symptom".
3. Ultimately, MSG allergy proved to be largely false.
4. MSG is found everywhere.
"Jewish Deli Syndrome" could be considered racist depending on the surrounding circumstance.
However, if there is set of ingredients that are unique to Jewish deli meat scientifically proven to cause problems for non-trivial percentage of population, then I would agree with you that it wouldn't (and shouldn't) be racist to call "Jewsih Deli Syndrome".
Evil bankers exist of lots of ethnicities and faiths but lots of people do (often but not always subconsciously) link greedy bankers and jewish people.
Just like using unhealthy or even dangerous additives is often linked to Chinese cuisine even though they're used just as much in tons of other cuisines as well.
Equating an existent stereotype with one which doesn't actually exist and saying "it's not that bad, I wouldn't be offended by that" is the false analogy. Because then it doesn't have the personal impact a prejudice or misconception you've actually been exposed to has.
A white person might say that if someone joked "all white people are lazy", they wouldn't be offended so a black or latino person shouldn't either. A black person is much more likely to be offended by "all black people are lazy" because that's something that's actually rooted in societal prejudices. Change it to "all white people are racist" and suddenly by their defensive tone, you'll note that you've hit a nerve and they're actually uncomfortable and/or offended.
"Chinese Restaurant Syndrome" has nothing to do with Chinese food being bad or unhealthy or dangerous. It's just that is is (was?) fairly common for there to be more MSG in Chinese Food. So it was a natural shorthand. It hadn't even occurred to me that someone would think it was racist before today.
I'm torn between finding this funny and finding this irritating. There's enough genuine problems that people face based on race, nationality, gender or sexual orientation that debates like this just make it harder to get people to take us seriously on things that matter.
It's pointless to try to tell people what is and isn't offensive. Normally, I err on the side of thinking that if somebody is offended, they must have reason to be and I should be sensitive to that, but in this case I had no idea what racism the author was referring to.
Not saying it's high on the prioritised list of racist tropes we need to be fighting, but yeah, it's there.
Maybe a better comparison would be "Jewish Bank Syndrome", which to me sounds a lot less harsh than "Jewish Banker Syndrome".