I've canned tomatoes using the oven instead of water bath. Back before it was deemed unsafe, my old ball blue book had instructions for it..I remember the "don't put the lids on the jars while they are in the oven".
Water bathing, let alone the “oven method”, is being “phased out” for canning tomatoes according to FDA, because too many “modern” tomatoes have had the necessary acidity levels bred out of them in favor of “sweetness”, making them a “low acid” food!
IF you’re used to using the oven method; still have the “officiall” instructions to refer to; and have high-acid heirlooms, etc., it’s PROBABLY okay for you to continue; but not so much for someone like Marcella, who is totally inexperienced.