Hello,
Yes, the case you detail can't be detected, because both values use the same data, a number. In both cases MS Excel sets a number, and the display format is set using the numFmtId attribute of the styles file:
https://c-rex.net/projects/samples/ooxml/e1/Part4/OOXML_P4_DOCX_numFmt_topic_ID0EHDH6.html
There's no method in the current stable version of phpxlsx to return this information. We have opened a task to add support.
Regards.