Judging by everything we've seen so far, I'd pick the latter.
? Tamriel is the centre of the universe, the hub of a mystical wheel. The sun is a big hole to another realm.
The laws of nature are semi sentient and can be persuaded to act differently or cease function alltogether.
That, by definition, makes it high fantasy.
That the derived, smaller scale world where we actually spend our time acts more or less like our world is good writing, as far as Im concerned, but it doesnt change the fact that the planets in the sky are actually Gods.
.. I mean.. even your own quote agrees with me..
"Low fantasy is a term used to describe a variety of works within differing sub-genres of fantasy fiction. The word "low" refers to the level of prominence of traditional fantasy elements within the work, and is not any sort of remark on the work's quality. Within the fantasy genre, low fantasy is often contrasted with
high fantasy, which typically takes place, partly or entirely, in a completely fictional setting and places an emphasis upon fantasy elements such as magic, monsters, and non-real literary devices. Low Fantasy works typically place relatively less emphasis on such fantasy elements and often take place within real-world environments, as opposed to entirely fictional settings."
Does that describe Tamriel? I think it does.