I don't think he did glide for 15 miles. He did though while gliding take out one last Stuka. Yeah right.Although the omitted words there are "from altitude"
The Dunkirk plane was at low level when he commenced his 15 mile glide - that wasn't feasible
Of a bigger note he could have turned back earlier. Great he carried on fighting as there was a bomber strafing people in the water so he made the decision to stay, but he lost this country an aircraft and pilot. Both which would have been needed "for tomorrow" and for the Battle of Britain.