It’s not undemocratic. If Johnson wasn’t such a coward then the final say on the 90 days would have been with the full House of Commons, i.e. by all our elected representatives. How this that undemocratic?
Because the other MPs in the House of Commons weren't chosen by the people of Uxbridge and South Ruislip to represent them. The other MPs have no right to tell those constituents that they cannot be represented for a whole 90 days by the person they chose.
There have to be limits on what a majority of the House of Commons is allowed to do. Else eg. any party with a majority could bar minority parties from participating in debates, or voting. It has to be a basic principle that MPs, even with a majority, cannot restrict the ability of other MPs to perform the task of representing the people who sent them there. The only thing they should be able to do in extremis is to refer the matter back to the relevant electorate.