If a Republican made a point of referring to President Obama as "Barack Hussein Obama" then that is a dog whistle, playing on the whole is he a muslim/his dad was a muslim thing while at face value just saying his full name.
Boris on the other hand has quite openly criticised the veil, he was straight up about it, there was no additional meaning to be implied. The fact some racists might have liked it isn't particularly relevant.
It's completely relevant if he's trying to divide the party.
It's not just me that sees it, either (I just googled "dog whistle burka"):
"Boris Johnson accused of ‘dog-whistle’ Islamophobia over burka remarks"
https://www.irishtimes.com/news/wor...tle-islamophobia-over-burka-remarks-1.3589047