Manuscript revision deadline: how strict are these [at Scientific Reports]

I had the same issue with Scientific Reports and I contacted the staff asking for an extension. Here is the answer:

Dear authors, Thank you for your email. This will not be a problem – we do appreciate that some revisions do take longer than others and we would be more than happy to accommodate an extension for you. Please submit your revised manuscript when you are ready.

These days, the publication venues are trying hard to maintain two things: (1) the quality-higher, (2) the turnaround time of the publication-lower. Both these parameters decide the popularity of the journal, which further attracts quality research from future authors.

The email that you have received focuses indirectly on the second parameter--the time from submission to acceptance. You could contribute to a small turnaround time by submitting the revision within 4 weeks. If you delay the process, then their turnaround time might get affected for the year (though one article may not impact much, it could be considered somewhat serious).

The above claim is validated by their second line in which they say "If you cannot send it within this time, please let us know so that we can close your file. In this event, we will still be happy to reconsider your paper at a later date as long as you haven’t submitted similar or related work elsewhere in the meantime."-- This is a strong statement (mark the bold text). So, now if you submit a revised version as a "new" submission, they don't have to put much effort and the article TAT will be as low as possible.