Sentence like “... are already described in section XYZ” are certainly not wrong, and, as already has been said, are not past tense. Although there is nothing wrong either with “... have already been described in section XYZ”, which is in a past tense. Indeed, the past tense indicates that the description you mention precedes the current page in your thesis, which the present tense does not.
I find this modern trend of writing scientific summaries in the present tense quite unnatural, and asking you to do this in a thesis seems perverse in the extreme. I always insisted that my students used tenses as follows:
Past tense: To describe what you did or thought (especially in the Summary and Results and Methods sections).
e.g. I therefore decided to investigate whatever… and conducted experiments in which…
Present tense: Universal ‘truths’ (especially in the Introduction and Conclusion)
e.g. The synthesis of proteins is controlled by…
As well as reading more naturally (note also the use of the first person singular rather than the passive) it avoids the ambiguity in a Summary of whether you are talking about your own work or that which someone else did before you (especially for sentences in the passive). I find that irritating, and it is good policy to avoid practices that might irritate your examiners (to which I would add in-group acronyms).
That’s my advice. Take it (if your supervisor will let your) or leave it. (But you could ask him why he wants you to write about past events in the present tense. I’d be interested to know.)