Yes, Sigil would not update a PI like the OP used if moved or the stylesheet was renamed. The OP could use a link tag immediately after the svg tag and that would be updated properly by Sigil after moves and name changes.
Something like:
Code:
<link xmlns="http://www.w3.org/1999/xhtml" href="../Styles/Style.css" rel="stylesheet" type="text/css" />"
Using PI's to link in things would probably not be supported by many e-readers if at all. Although it works in Sigil now, after moving or renaming the stylesheet or moving the svg file relative to stylesheet, that PI would not be updated automatically while the link approach would be.