I think you may be over hyping this decision.
A parliamentary private secretary job is unpaid, and involves as lttle or as much work as the MP wants to put it. They are there to report to Ministers what they hear in parliament from other parliamentarians. This is so Ministers can guage their own popularity (or lack of) amongst there colleagues. It is basically feedback.
Jenkyns had only been doing her PPS role for a few weeks. But part of the Code is supposed to be loyalty to the Government and Prime Minister. It is this muzzling aspect that Jenkyns is wanting to shed by resigning. She wants to criticise May (for her poor performance on Brexit) but she is unable to do whilst she is a PPS.
If anything, this is further evidence that May has lost control of her party, if a PPS would rather resign than support the Government she belongs to.