Since you have solved it, I allow myself to ask if my problem was calmag heat too? https://rollitup.org/t/dead-spots-on-lower-leaves.1000248/#post-15165460
From the article you quote: Unlike most other deficiencies that cause yellowing of the leaves, a sulfur deficiency will start at the back of the leaf and move it’s way forward as opposed to starting at the tips.
I can't spot that on his pics, can you?