Seedlings 10 days old- a log

[stinkyattic]

Someone (sigh ) told me that if I put the soil over too much of the stem it'd rott, is that BS?) l

Nope. The cotyledons are the first little round seed leaves that come out. The entire stem below them can functionally be considered almost an extension of the taproot, and is as happy underground as above it. If you put soil over a part of the stem ABOVE the seed-leaves, which is designed to be only ABOVE ground, then you can indeed get rot.