There's currently no means of exporting InDesign lines or shapes as SVG files. If you place SVGs into your InDesign document, then those files will remain as SVGs when you export with in5.
If you'd like lines or shapes created in InDesign to appear more crisp when exporting them as PNGs, then try setting the Image Quality setting to High Definition (144 ppi) in the Advanced section of the Export HTML5 with in5 dialog.
You could also create an Object Style to apply to your lines to make them export at an even higher resolution (150 or 300 ppi). Here's more information:
I checked with Justin about exporting SVGs. He said that in5 does not produce SVG from lines and shapes, since PNG usually generates smaller files. The only exception is a simple circle with a solid fill. in5 creates an SVG in this case because it produces a smaller file size in the output.
I would add that if you're placing multiple lines that are the same (size, color, etc.), if you have the Gold plan or higher, and if you go to the Advanced section of the Export HTML5 with in5 dialog (in5 > Export HTML5 with in5...) and set Image Quality to Enhanced, then in5 will consolidate the images for you so that the document only references the one image (or fewer images if you have different sizes of lines that repeat). Here's more on reducing image size: