When canning any kind of veggies this is true and you should use a pressure canner (my best friend does not and says she has always had great success, but the one time I tried it, I was petrified of the results)
However, most fruits do not need a pressure canner, they can be canned in a hot water bath. At the same time, most recipes call for use of a heavy syrup (1:1) is also unnecessary. I always used just a VERY light syrup of 1 cup of sugar to to 8 cups of water. My sister just sprinkles in 1 Tablespoon sugar per quart jar. Another lady I know does not use any sugar at all. So you see, several people doing the same food may have different ways of doing it and all get great results.
The original question was about tomatoes which is technically a fruit. They do not need a pressure canner. I would never add lemon juice to tomatoes anyway because it will impact the flavour of the finished product. If you MUST use something, use citric adic crystals available where most canning supplies are sold......
I learned how to can tomatoes from my Mother in law and she never added anything to her tomatoes.
If it would make you feel better to reprocess those tomatoes, then go ahead and redo..... You don't need to but it would be a small price to pay for peace of mind.