You definitely want to use iRT (retention time normalization) with DIA/SWATH data. Performance without it will be very poor, but we have recently (in 2020) done work to make it relatively easy to use endogenous peptides as the iRT anchor peptides. So, in that sense, your colleague is right that Skyline does not require you to inject an iRT standard mix. Though, Skyline also works very well with all of the industry-standard retention time calibration mixes that we know of.
Nick has pointed you to our existing tutorial on iRT. Though, the DIA/SWATH tutorial is also a good place to look. The tutorial uses the Biognosys iRT mix, but you can now ask Skyline to automatically calibrate iRT and it will use endogenous peptides, if it does not detect one of the known RT standard mixes.
Hope this helps. Thanks for posting your question to the Skyline support board.