I could reproduce the issue, and couldn't find a way to work around the issue using OLE. So, the only way to call OLEobjects.add is to convert the whole code into VBscript, and call it via DOI (method EXECUTE_MACRO of I_OI_SCRIPT_COLLECTION - search the forum and the SAP library Desktop Office Integration).
PS: 0 for false, and 1 for true are correct assumptions. Moreover, I did tests with another method whose first parameter was optional, and I noticed that doing "worksheet.checkSpelling ,true" (second parameter "ignoreUpperCase") with only EXPORTING #2 = 1 was unsufficient, it was working only with EXPORTING #1 = '' #2 = 1. So, I came to the conclusion that all first parameters are to be passed, even if not used, which means that OLEobjects.add may never work because OLEobjects.add '', 'filepath', false, false fails in VBA (the first parameter ClassType must not be passed).