as ever, it’s worth reading a lawyer’s exact words rather than simply accepting assurances about intent. Here is the precise wording (with my emphasis):

‘…a Motion in connection with the United Kingdom’s withdrawal from the European Union in the name of at least 300 Members of the House elected to the House as members of at least five parties and including at least 10 Members elected to the House as members of the party in Government shall stand as the first item of business;’

So such a motion would not require the support of ten MPs from the governing party, only ten MPs ‘elected to the House as members’ of such a party. In other words, MPs who had defected, or resigned the Whip, or been expelled from the governing party would still count as lending support from the Government benches. How convenient.