This is what a recording session reveal when opening a sample tab-delimited file and using the default open wizard Excel uses:
tell application "Microsoft Excel"
OpenText "bootdrive:Users:username:Desktop:testfile.txt" Origin xlMacintosh StartAtRow 1 DataType xlDelimited TextQualifier xlDoubleQuote FieldInfo {{1.0, 1.0}, {2.0, 1.0}, {3.0, 1.0}, {4.0, 1.0}, {5.0, 1.0}, {6.0, 1.0}, {7.0, 1.0}, {8.0, 1.0}, {9.0, 1.0}, {10.0, 1.0}, {11.0, 1.0}, {12.0, 1.0}, {13.0, 1.0}, {14.0, 1.0}, {15.0, 1.0}, {16.0, 1.0}, {17.0, 1.0}} with TabDelimiter without TreatConsecutiveDelimitersAsOne, SemicolonDelimiter, CommaDelimiter, SpaceDelimiter and Other
end tell
Credit to Microsoft, their AppleScript syntax is wonky, but if you use Record, it’s very, very thorough. Usually it’s a bit more than you bargained for.
Looks like this works well enough:
tell application "Microsoft Excel"
OpenText "bootdrive:Users:username:Desktop:testfile.txt" with TabDelimiter
end tell
Everyone always forgets that Excel is one of the few “recordable” apps around. 