please replace the angle brackets by square brackets or "<_" and "_>" or something - otherwise they do not display in the post. As far as I can see from the source code of this page, the line is
[httpruntime usefullyqualifiedredirecturl="true" targetframework="4.7.2" shutdowntimeout="36000" requestvalidationmode="2.0" requestpathinvalidcharacters="<,>,*,%,:,\,?" requestlengthdiskthreshold="512000" maxurllength="2048" maxrequestlength="512000" fcnmode="Single" executiontimeout="360000" enableversionheader="false"][/httpruntime]
The MaxRequestLength is the overall total length the request cannot exceed. The RequestLengthDiskThreshold should be less than the MaxRequestLength value and indicates at what point (or "threshold") the request will begin to be buffered transparently onto disk. The value of RequestLengthDiskThreshold must not exceed the value of MaxRequestLength.
In your example, the MaxRequestLength is 512000 kB (which should be 524288, that means 512 MB, but that is not too important). The RequestLengthDiskThreshold is 512000 b (which should also be 524288, which means 512 kB). To be honest, the second value is really high, I normally lower the default value (which is 80 kB = 81920) to 4096 (which is the default allocation unit size of an NTFS disk). That helps to not chew up too much memory in IIS and on the server, so buffering to disk does help keep memory management under control. Also note: the value for MaxRequestLength is in kB, as the value for RequestLengthDiskThreshold is in byte.
So far for the theory... :-)
In DNN it seems that values for the file size upload are limited to 28 MB - see Settings >> Security >> More >> More Security Settings >> "Max Upload Size". The error message if you enter a value greater than 28 or less than 6 says "Maximum upload size must be between 12 and 28". Changing this value changes the value in the web.config, as the help text suggests ("This is a global setting. Changes to this setting will affect all of your sites.").
And also if you change the setting in your web.config to a higher value - this is ignored by DNN when you upload a file.
The only way to avoid this behaviour would be a module that ignores the DNN settings (and maybe as well the web.config settings) and writes directly to the file system. There used to be one (SwfFileUploader by Oliver Hine), but this is no longer available and supported (as Flash is reaching the end of its life).
To be honest: I think the maximum upload limit should be changed asap. It was not like this in former DNN versions, and it should be in an administrator's or host's responsibility to set it as it's needed. For instance: in an intranet, this could be a high value without any problem.