I don't think anyone would argue that Spurs don't have a quality first team. The argument is that beyond that first team they are (relatively) threadbare.
For example, let's take their 3-1 loss to Wolves a few weeks ago as one of the games they are accused of bottling. On the bench that day they they had Rose, N'Koudou, Walker-Peters, Foyth, Gazzaniga, Moura and Skipp. Now, with all the best will in the world, that is utter shite. This has two effects: 1) The quality players in their starting 11 are exhausted from being overplayed due to the lack of appropriate cover and 2) they have next to nothing to bring on to change or even see out a game.
To look at that and then say "well, mental fragility must be the reason they lost" is odd. The reason they lost is because a team that lacking in depth is supposed to lose sometimes. The fact that they managed to get ahead of City (who usually have two of Mahrez, Sane, Silva or Sterling on their bench every week) at any point is the story there, not that they couldn't keep it up.