It's just a tiny mistake that most Germans are used to hearing Americans make all the time (see also Porsche which is also not pronounced porsh, nor por-shay, but porsh-eh) and will politely ignore, but since this aims to be an educational video, should be pointed out to be slightly incorrect
As another German, I can confirm that the "first e in mesmer" way is how Germans would pronounce it. See for example 11seconds into this German video also officially from SUSE's YouTube channel - a SUSE employee and German native speaker who is moderating a series of talks is using that pronunciation.
It's just a tiny mistake that most Germans are used to hearing Americans make all the time (see also Porsche which is also not pronounced porsh, nor por-shay, but porsh-eh) and will politely ignore, but since this aims to be an educational video, should be pointed out to be slightly incorrect