. . . . Does exporting video as "Streamable" reduce it's quality in any way? . . . . would ticking the "Streamable" box change anything
No and yes, respectively.
Making a file streamable means important pieces of information are put into the header, think 'information block', at the start of the video file, and it 'informs' the player about various video parameters so video can start playing it as it downloads, instead of waiting for the whole file to have been downloaded before playing.
However, the website you upload to, may re-encode the video to different resolutions and/or compress it more than that which was uploaded. You would have no control over this if it is a 3rd party website such as Youtube, Vimeo etc.